So, I'm developing some software, and trying to keep myself using TDD and other best practices.
I'm trying to write tests to define the classes and repository.
Let's say I have the classes, Customer, Order, OrderLine.
Now, do I create the Order class as something like
abstract class Entity {
int ID { get; set; }
}
class Order : Entity {
Customer Customer { get; set; }
List<OrderLine> OrderLines { get; set; }
}
Which will serialize nice, but, if I don't care about the OrderLines, or Customer details is not as lightweight as one would like. Or do I just store IDs to items and add a function for getting them?
class Order : Entity {
int CustomerID { get; set; }
List<OrderLine> GetOrderLines() {};
}
class OrderLine : Entity {
int OrderID { get; set; }
}
And how would you structure the repository for something like this?
Do I use an abstract CRUD repository with methods GetByID(int), Save(entity), Delete(entity) that each items repository inherits from, and adds it's own specific methods too, something like this?
public abstract class RepositoryBase<T, TID> : IRepository<T, TID> where T : AEntity<TID>
{
private static List<T> Entities { get; set; }
public RepositoryBase()
{
Entities = new List<T>();
}
public T GetByID(TID id)
{
return Entities.Where(x => x.Id.Equals(id)).SingleOrDefault();
}
public T Save(T entity)
{
Entities.RemoveAll(x => x.Id.Equals(entity.Id));
Entities.Add(entity);
return entity;
}
public T Delete(T entity)
{
Entities.RemoveAll(x => x.Id.Equals(entity.Id));
return entity;
}
}
What's the 'best practice' here?
Entities
Let's start with the Order entity. An order is an autonomous object, which isn't dependent on a 'parent' object. In domain-driven design this is called an aggregate root; it is the root of the entire order aggregate. The order aggregate consists of the root and several child entities, which are the OrderLine entities in this case.
The aggregate root is responsible for managing the entire aggregate, including the lifetime of the child entities. Other components are not allowed to access the child entities; all changes to the aggregate must go through the root. Also, if the root ceases to exist, so do the children, i.e. order lines cannot exist without a parent order.
The Customer is also an aggregate root. It isn't part of an order, it's only related to an order. If an order ceases to exist, the customer doesn't. And the other way around, if a customer ceases to exist, you'll want to keep the orders for bookkeeping purposes. Because Customer is only related, you'll want to have just the CustomerId in the order.
class Order
{
int OrderId { get; }
int CustomerId { get; set; }
IEnumerable<OrderLine> OrderLines { get; private set; }
}
Repositories
The OrderRepository is responsible for loading the entire Order aggregate, or parts of it, depending on the requirements. It is not responsible for loading the customer. If you need the customer, load it from the CustomerRepository, using the CustomerId from the order.
class OrderRepository
{
Order GetById(int orderId)
{
// implementation details
}
Order GetById(int orderId, OrderLoadOptions loadOptions)
{
// implementation details
}
}
enum OrderLoadOptions
{
All,
ExcludeOrderLines,
// other options
}
If you ever need to load the order lines afterwards, you should use the tell, don't ask principle. Tell the order to load its order lines, and which repository to use. The order will then tell the repository the information it needs to know.
class Order
{
int OrderId { get; }
int CustomerId { get; set; }
IEnumerable<OrderLine> OrderLines { get; private set; }
void LoadOrderLines(IOrderRepository orderRepository)
{
// simplified implementation
this.OrderLines = orderRepository.GetOrderLines(this.OrderId);
}
}
Note that the code uses an IOrderRepository to retrieve the order lines, rather than a separate repository for order lines. Domain-driven design states that there should be a repository for each aggregate root. Methods for retrieving child entities belong in the repository of the root and should only be accessed by the root.
Abstract/base repositories
I have written abstract repositories with CRUD operations myself, but I found that it didn't add any value. Abstraction is useful when you want to pass instances of subclasses around in your code. But what kind of code will accept any BaseRepository implementation as a parameter?
Also, the CRUD operations can differ per entity, making a base implementation useless. Do you really want to delete an order, or just set its status to deleted? If you delete a customer, what will happen to the related orders?
My advice is to keep things simple. Stay away from abstraction and generic base classes. Sure, all repositories share some kind of functionality and generics look cool. But do you actually need it?
I would divide my project up into the relevant parts. Data Transfer Objects (DTO), Data Access Objects (DAO). The DTO's I would want to be as simple as possible, terms like POJO (Plain Old Java Object) and POCO (Plain Old C Object) are used here, simply put they are container objects with very little if any functionality built into them.
The DTO's are basically the building blocks to the whole application, and will marry up the layers. For every object that is modeled in the system, there should be at least one DTO. How you then put these into collections is entirely up to the design of the application. Obviously there are natural One to many relationships floating around, such as Customer has many Orders. But the fundamentals of these objects are what they are. For example, an order has a relationship with a customer, but can also be stand alone and so needs to be separate from the customer object. All Many to Many Relationships should be resolved down into One to Many relationships which is easy when dealing with nested classes.
Presumably there should be CRUD objects that appear within the Data Access Objects category. This is where it gets tricky as you have to manage all the relationships that have been discovered in design and the lifetime models of each. When fetching DTO's back from the DAO the loading options are essential as this can mean the difference between your system running like a dog from over eager loading, or high network traffic from fetching data back and fourth from your application and the store by lazy loading.
I won't go into flags and loading options as others here have done all that.
class OrderDAO
{
public OrderDTO Create(IOrderDTO order)
{
//Code here that will create the actual order and store it, updating the
flelds in the OrderDTO where necessary. One being the GUID field of the new ID.
I stress guid as this means for better scalability.
return OrderDTO
}
}
As you can see the OrderDTO is passed into the Create Method.
For the Create Method, when dealing with brand new nested Objects, there will have to be some code dealing with the marrying up of data that has been stored, for example a customer with old orders, and a new order. The system will have to deal with the fact that some of the operations are update statements, whilst others are Create.
However one piece of the puzzle that is always missed is that of multi-user environments where DTO's (plain Objects) are disconnected from the application and returned back to the DAO for CRUD. This usually involves some Concurrency Control which can be nasty and can get complicated. A simple mechanism such as DateTime or Version number works here, although when doing crud on a nested object, you must develop the rules on what gets updated and in what order, also if an update fails concurrency, you have to decide on whether you fail all the operation or partial.
Why not create separate Order classes? It sounds to me like you're describing a base Order object, which would contain the basic order and customer information (or maybe not even the customer information), and a separate Order object that has line items in it.
In the past, I've done as Niels suggested, and either used boolean flags or enums to describe optionally loading child objects, lists, etc. In Clean Code, Uncle Bob says that these variables and function parameters are excuses that programmers use to not refactor a class or function into smaller, easier to digest pieces.
As for your class design, I'd say that it depends. I assume that an Order could exist without any OrderLines, but could not exist without a Customer (or at least a way to reference the customer, like Niels suggested). If this is the case, why not create a base Order class and a second FullOrder class. Only FullOrder would contain the list of OrderLines. Following that thought, I'd create separate repositories to handle CRUD operations for Order and FullOrder.
If you are interested in domain driven design (DDD) implementation with POCOs along with explanations take a look at the following 2 posts:
http://devtalk.dk/2009/06/09/Entity+Framework+40+Beta+1+POCO+ObjectSet+Repository+And+UnitOfWork.aspx
http://www.primaryobjects.com/CMS/Article122.aspx
There is also a project that implements domain driven patterns (repository, unit of work, etc, etc) for various persistence frameworks (NHibernate, Entity Frameworks, etc, etc) called NCommon
Related
This is a common requirement in many web-based projects: an entity has to show information to another related entity. For example, a book in an e-commerce site has to show relevant information about its author.
Let say I model both the book and author as an entity, how should I implement a feature which display a book and its author's information on the same page.
I can make a call to the BookRepo to retrieve the book's information, and then another call to the AuthorRepo to retrieve the author, using the authorid inside the book entity. This is 2 queries
I can write a query where I join the Book and Author tables together and retrieve both information in 1 query. But which repo does this query goes? Does this break DDD because I am assuming details about the Book and Author entity?
Which is the 'best practices', and what are other ways I can approach this problem?
(I am assuming using the use of standard SQL queries [such as PHP + MySQL], since in EF 4 you would define associations between Book and Author which would solve the problem rather easily).
There is no silver bullet solution to this, but you have a few options.
Your first proposed solution of making a call to two repositories is perfectly valid and happens all the time in practice. For example, it takes over a hundred different services to render an Amazon product page. Each service is responsible for providing data specific to its bounded context. You can create a service called something like BookService which calls the two repositories to return a reporting object or a DTO that has all the data that you need for the particular view. If you feel that performance is due to two repository calls is going to be an issue, you can employ caching, or CQRS to create appropriate read models, but don't jump to those solutions prematurely.
But which repo does this query goes?
I would just add it to the BookRepository, or even a whole new repository called BookDetailsReportingRepository, perhaps a method called GetBookDetails. This method would not return an editable entity, but a reporting object which is a projection of values from multiple entities.
Does this break DDD because I am assuming details about the Book and Author entity?
This does not violate DDD and in my opinion makes it easier to apply. Just regard as the data returned by the aforementioned repository as reporting objects, not entities.
But even though you don't seems to use ORM's you probably have tto populate your entities from your SQL querys and your entities has to relate to each other in some way (collections, navigation properties etc...).
If your domain contains entities that have no associations between each other, I wouldn't call it DDD since you lack some important ingredients like aggregates, value objects, bi-/unidirectional relationships.
But what do I know :-) Maybe you have made your puzzle well and this last piece is to merge entities into a "view" that can be useful for your clients.
Since repositories normally operates on an aggregate-root entity you can have repository methods like ListBooksByAuthor (BookRepository) or ListAuthors (AuthorRepository).
When you want to display complex data from many different aggregates in a web page I recommend using Data Transfer Objects. Let that DTO object be unique for that page or Use Case and being a "view" that displays all (or most of the) data that web page needs.
I also recommend NOT using DTO's everywhere unless you're using web services. Using DTO's together gives both pros and cons. Together with a service layer it gives you a nive anti corruption layer and also gives you the place to inject Book and Author repository. Then from service layer you can assemble and reassemble DTO's (look at AutoMApper or similar...helps you a lot).
BUT to much DTO's everywhere also gives you overhead in maintenance of the application. It adds another layer to maintain.
I prefer to just use it for certain clients/web pages.
I hope you understand what I'm trying to explain :-)
If you look at this page it describes two ways to load and relate the aggregate roots. Linking this back to your example:
The Book class would encapsulate the relevant Author information as a Value Type so when the Book information is displayed on the web page it has all the information on the author it needs. If the users decides to view more information on the Author they can by following a link to an Author Page (whatever the requirement is).
If you have a service method called FindBookByTitle then
loading the Book entity would then load the relevant author information from the BookRepo.
class Author
{
public Author(int authorID, FullName name) { }
int AuthorID { get; }
FullName Name { get; }
List<BookDetails> AuthoredBooks { get; set; }
}
class BookDetails
{
public BookDetails(int ID, string title) { }
int BookID { get; }
string Title { get; }
}
class Book
{
public Book(int ID, string Title) { }
int BookID { get; }
string Title { get; }
List<AuthorDetails> Writers { get; set; }
}
class AuthorDetails
{
public AuthorDetails(int ID, FullName name) { }
int AuthorID { get; }
public FullName fullName { get; }
}
class FullName
{
public FullName(string name, string surname) { }
public string Name { get; }
public string Surname { get; }
}
I am in midst of developing an application using Entity Framework code first 4.1 and MVC3. Here are three entities that I have, basically, State => City => Locality.
public class State {
public virtual List<City> Cities { get; set; }
}
public class City {
public virtual List<Locality> Localities { get; set; }
public virtual State State { get; set; }
}
public class Locality {
public virtual City City { get; set; }
}
It can be seen that I am using bi-directional relationship for all three entities. Json does not allow that, so I am somewhat frustrated. Basically, I need to navigate in either direction. For example given a City I want to be able to locate State to which it belongs easily.
I scanned the web and came across a couple of workarounds, but none of them suits my scenario. The first was to make the relationship unidirectional(who will do that !) and I don't want that. Other was to introduce ViewModel with only the properties that I need, but then that would mean duplicate code if I need to use all the fields of Entity regularly. Also my controller will be flooded with those properties. So I don't like that too.
I was thinking that this was just basic stuff, but now I am struggling to find a workable solution. If anyone has a better alternative(perhaps something in MVC3), please help me out.
You could try using JavaScriptSerializer directly and registering your own converter to control the serialization process.
I am not an ASP expert, but I think the solution might be similiar to what I woudl do in Java, Groovy or python or any other language.
The best solution I could propose is to make City.Localities a kind of "transient" (in Java terms) field - i.e. don't serialize it, but update it at loading time (when you build the structure). This can be encapsulated in setter for City of the Locality class.
So in Locality.setCity, in the set method you should call (city->localities.append(this) (whatever language you write it in). This way it will become a "runtime cache" of City->Localities which will be build once during loading.
The problem appears to be a native issue with the DataContractJsonSerializer support for Entity types. In short, Entities that have relationships (i.e. two-way) with other Entity types cannot be serialized through Json. In your example: a State table connected to a Cities table will not translate well into Json serializing because a State may have many cities and a City is associated with a State.
One quick solution would be anonymous object projection.
Something like this example:
public JsonResult Orders()
var results = from Ord in databaseContext.Orders
select new
{
OrderID = Ord.ID,
OrderTitle = Ord.Name,
OrderDate = Ord.OrderDate
}
return Json(results);
}
For reference, see this: Serializing Entity Framework Objects into JSON Using ASP.Net MVC
I'm trying to decide on the best pattern for data access in my MVC application.
Currently, having followed the MVC storefront series, I am using repositories, exposing IQueryable to a service layer, which then applies filters. Initially I have been using LINQtoSQL e.g.
public interface IMyRepository
{
IQueryable<MyClass> GetAll();
}
Implemented in:
public class LINQtoSQLRepository : IMyRepository
{
public IQueryable<MyClass> GetAll()
{
return from table in dbContext.table
select new MyClass
{
Field1 = table.field1,
... etc.
}
}
}
Filter for IDs:
public static class TableFilters
{
public static MyClass WithID(this IQueryable<MyClass> qry, string id)
{
return (from t in qry
where t.ID == id
select t).SingleOrDefault();
}
}
Called from service:
public class TableService
{
public MyClass RecordsByID(string id)
{
return _repository.GetAll()
.WithID(id);
}
}
I ran into a problem when I experimented with implementing the repository using Entity Framework with LINQ to Entities. The filters class in my project contains some more complex operations than the "WHERE ... == ..." in the example above, which I believe require different implementations depending on the LINQ provider. Specifically I have a requirement to perform a SQL "WHERE ... IN ..." clause. I am able to implement this in the filter class using:
string[] aParams = // array of IDs
qry = qry.Where(t => aParams.Contains(t.ID));
However, in order to perform this against Entity Framework, I need to provide a solution such as the BuildContainsExpression which is tied to the Entity Framework. This means I have to have 2 different implementations of this particular filter, depending on the underlying provider.
I'd appreciate any advice on how I should proceed from here.
It seemed to me that exposing an IQueryable from my repository, would allow me to perform filters on it regardless of the underlying provider, enabling me to switch between providers if and when required. However the problem I describe above makes me think I should be performing all my filtering within the repositories and returning IEnumerable, IList or single classes.
Many thanks,
Matt
This is a very popular question. One that I constantly ask myself. I've always felt it best to return IEnumerable rather than IQueryable from a repository.
The purpose of a repository is to encapsulate the database infrastructure so the client need not worry about the data source. However, if you return IQueryable you are at the mercy of the consumer as to what kind of query will get run against your db, and whether they will do something that the LINQ provider doesn't support.
Take paging for example. Lets say you have a Customer entity and your database could have hundreds of thousands of customers. Which code would you rather have your client write?
var customers = repos.GetCustomers().Skip(skipCount).Take(pageSize).ToList();
OR
var customers = repos.GetCustomers(pageIndex, pageSize);
In the first approach you make it impossible for the repository to restrict the number of records retrieved from the data source. Also, your consumer has to calculate the skipCount.
In the second approach you provide a more coarse grained interface to your client. Now your repository can enforce some constraints on the pageSize in order to optimize the query. You also encapsulate the calculation of the skipCount.
However, that being said, in your situation your client is your service. So I suppose the question really comes down to a separation of concerns. Where is it better to perform such validation logic? Well that answer may very well be "in the service". But what about the answer to "Where is it better to contain query logic?". To me the answer is clearly "The Repository". That is its intended area of expertise.
I am new to domain models, POCO and DDD, so I am still trying to get my head around a few ideas.
One of the things I could not figure out yet is how to keep my domain models simple and storage-agnostic but still capable of performing some queries over its data in a rich way.
For instance, suppose that I have an entity Order that has a collection of OrdemItems. I want to get the cheapest order item, for whatever reason, or maybe a list of order items that are not currently in stock. What I don't want to do is to retrieve all order items from storage and filter later (too expensive) so I want to end up having a db query of the type "SELECT .. WHERE ITEM.INSTOCK=FALSE" somehow. I don't want to have that SQL query in my entity, or any variation of if that would tie me into a specific platform, like NHibernate queries on Linq2SQL. What is the common solution in that case?
Entities are the "units" of a domain. Repositories and services reference them, not vice versa. Think about it this way: do you carry the DMV in your pocket?
OrderItem is not an aggregate root; it should not be accessible through a repository. Its identity is local to an Order, meaning an Order will always be in scope when talking about OrderItems.
The difficulty of finding a home for the queries leads me to think of services. In this case, they would represent something about an Order that is hard for an Order itself to know.
Declare the intent in the domain project:
public interface ICheapestItemService
{
OrderItem GetCheapestItem(Order order);
}
public interface IInventoryService
{
IEnumerable<OrderItem> GetOutOfStockItems(Order order);
}
Declare the implementation in the data project:
public class CheapestItemService : ICheapestItemService
{
private IQueryable<OrderItem> _orderItems;
public CheapestItemService(IQueryable<OrderItem> orderItems)
{
_orderItems = orderItems;
}
public OrderItem GetCheapestItem(Order order)
{
var itemsByPrice =
from item in _orderItems
where item.Order == order
orderby item.Price
select item;
return itemsByPrice.FirstOrDefault();
}
}
public class InventoryService : IInventoryService
{
private IQueryable<OrderItem> _orderItems;
public InventoryService(IQueryable<OrderItem> orderItems)
{
_orderItems = orderItems;
}
public IEnumerable<OrderItem> GetOutOfStockItems(Order order)
{
return _orderItems.Where(item => item.Order == order && !item.InStock);
}
}
This example works with any LINQ provider. Alternatively, the data project could use NHibernate's ISession and ICriteria to do the dirty work.
Domain objects should be independent of storage, you should use the Repostiory pattern, or DAO to persist the objects. That way you are enforcing separation of concerns, the object itself should not know about how it is stored.
Ideally, it would be a good idea to put query construction inside of the repository, though I would use an ORM inside there.
Here's Martin Fowler's definition of the Repository Pattern.
As I understand this style of design, you would encapsulate the query in a method of an OrderItemRepository (or perhaps more suitably OrderRepository) object, whose responsibility is to talk to the DB on one side, and return OrderItem objects on the other side. The Repository hides details of the DB from consumers of OrderItem instances.
I would argue that it doesn't make sense to talk about "an Order that contains only the OrderItems that are not in stock". An "Order" (I presume) represents the complete list of whatever the client ordered; if you're filtering that list you're no longer dealing with an Order per se, you're dealing with a filtered list of OrderItems.
I think the question becomes whether you really want to treat Orders as an Aggregate Root, or whether you want to be able to pull arbitrary lists of OrderItems out of your data access layer as well.
You've said filtering items after they've come back from the database would be too expensive, but unless you're averaging hundreds or thousands of OrderItems for each order (or there's something else especially intensive about dealing with lots of OrderItems) you may be trying to optimize prematurely and making things more difficult than they need to be. I think if you can leave Order as the aggregate root and filter in your domain logic, your model will be cleaner to work with.
If that's genuinely not the case and you need to filter in the database, then you may want to consider having a separate OrderItem repository that would provide queries like "give me all of the OrderItems for this Order that are not in stock". You would then return those as an IList<OrderItem> (or IEnumerable<OrderItem>), since they're not a full Order, but rather some filtered collection of OrderItems.
In the service layer.
I seem to have gotten myself into a bit of a confusion of this whole DDD\LinqToSql business. I am building a system using POCOS and linq to sql and I have repositories for the aggregate roots.
So, for example if you had the classes Order->OrderLine you have a repository for Order but not OrderLine as Order is the root of the aggregate. The repository has the delete method for deleting the Order, but how do you delete OrderLines?
You would have thought you had a method on Order called RemoveOrderLine which removed the line from the OrderLines collection but it also needs to delete the OrderLine from the underlying l2s table. As there isnt a repository for OrderLine how are you supposed to do it?
Perhaps have specialized public repostories for querying the roots and internal generic repositories that the domain objects actually use to delete stuff within the aggregates?
public class OrderRepository : Repository<Order> {
public Order GetOrderByWhatever();
}
public class Order {
public List<OrderLines> Lines {get; set;} //Will return a readonly list
public RemoveLine(OrderLine line) {
Lines.Remove(line);
//************* NOW WHAT? *************//
//(new Repository<OrderLine>(uow)).Delete(line) Perhaps??
// But now we have to pass in the UOW and object is not persistent ignorant. AAGH!
}
}
I would love to know what other people have done as I cant be the only one struggling with this.... I hope.... Thanks
You call the RemoveOrderLine on the Order which call the related logic. This does not include doing changes on the persisted version of it.
Later on you call a Save/Update method on the repository, that receives the modified order. The specific challenge becomes in knowing what has changed in the domain object, which there are several options (I am sure there are more than the ones I list):
Have the domain object keep track of the changes, which would include keeping track that x needs to be deleted from the order lines. Something similar to the entity tracking might be factored out as well.
Load the persisted version. Have code in the repository that recognizes the differences between the persisted version and the in-memory version, and run the changes.
Load the persisted version. Have code in the root aggregate, that gets you the differences given an original root aggregate.
First, you should be exposing Interfaces to obtain references to your Aggregate Root (i.e. Order()). Use the Factory pattern to new-up a new instance of the Aggregate Root (i.e. Order()).
With that said, the methods on your Aggregate Root contros access to its related objects - not itself. Also, never expose a complex types as public on the aggregate roots (i.e. the Lines() IList collection you stated in the example). This violates the law of decremeter (sp ck), that says you cannot "Dot Walk" your way to methods, such as Order.Lines.Add().
And also, you violate the rule that allows the client to access a reference to an internal object on an Aggregate Root. Aggregate roots can return a reference of an internal object. As long as, the external client is not allowed to hold a reference to that object. I.e., your "OrderLine" you pass into the RemoveLine(). You cannot allow the external client to control the internal state of your model (i.e. Order() and its OrderLines()). Therefore, you should expect the OrderLine to be a new instance to act upon accordingly.
public interface IOrderRepository
{
Order GetOrderByWhatever();
}
internal interface IOrderLineRepository
{
OrderLines GetOrderLines();
void RemoveOrderLine(OrderLine line);
}
public class Order
{
private IOrderRepository orderRepository;
private IOrderLineRepository orderLineRepository;
internal Order()
{
// constructors should be not be exposed in your model.
// Use the Factory method to construct your complex Aggregate
// Roots. And/or use a container factory, like Castle Windsor
orderRepository =
ComponentFactory.GetInstanceOf<IOrderRepository>();
orderLineRepository =
ComponentFactory.GetInstanceOf<IOrderLineRepository>();
}
// you are allowed to expose this Lines property within your domain.
internal IList<OrderLines> Lines { get; set; }
public RemoveOrderLine(OrderLine line)
{
if (this.Lines.Exists(line))
{
orderLineRepository.RemoveOrderLine(line);
}
}
}
Don't forget your factory for creating new instances of the Order():
public class OrderFactory
{
public Order CreateComponent(Type type)
{
// Create your new Order.Lines() here, if need be.
// Then, create an instance of your Order() type.
}
}
Your external client does have the right to access the IOrderLinesRepository directly, via the interface to obtain a reference of a value object within your Aggregate Root. But, I try to block that by forcing my references all off of the Aggregate Root's methods. So, you could mark the IOrderLineRepository above as internal so it is not exposed.
I actually group all of my Aggregate Root creations into multiple Factories. I did not like the approach of, "Some aggregate roots will have factories for complex types, others will not". Much easier to have the same logic followed throughout the domain modeling. "Oh, so Sales() is an aggregate root like Order(). There must be a factory for it too."
One final note is that if have a combination, i.e. SalesOrder(), that uses two models of Sales() and Order(), you would use a Service to create and act on that instance of SalesOrder() as neither the Sales() or Order() Aggregate Roots, nor their repositories or factories, own control over the SalesOrder() entity.
I highly, highly recommend this free book by Abel Avram and Floyd Marinescu on Domain Drive Design (DDD) as it directly answers your questions, in a shrot 100 page large print. Along with how to more decouple your domain entities into modules and such.
Edit: added more code
After struggling with this exact issue, I've found the solution. After looking at what the designer generates with l2sl, I realized that the solution is in the two-way associations between order and orderline. An order has many orderlines and an orderline has a single order. The solution is to use two way associations and a mapping attribute called DeleteOnNull(which you can google for complete info). The final thing I was missing was that your entity class needs to register for Add and Remove events from the l2s entityset. In these handlers, you have to set the Order association on the order line to be null. You can see an example of this if you look at some code that the l2s designer generates.
I know this is a frustrating one, but after days of struggling with it, I've got it working.
As a follow up....
I have switched to using nhibernate (rather than link to sql) but in effect you dont need the repos for the OrderLine. If you just remove the OrderLine from the collection in Order it will just delete the OrderLine from the database (assuming you have done your mapping correctly).
As I am swapping out with in-memory repositories, if you want to search for a particular order line (without knowing the order parent) you can write a linq to nhibernate query that links order to orderline where orderlineid = the value. That way it works when querying from the db and from in memory. Well there you go...