I'm a new to Grails and have started a project, but I'm having troubles finding out what is wrong:
The project is already connected to my database (SQL) and it has a few many-to-many to relationship with more than 1 "parameter", like this:
static hasMany = [rules:AvaliateRules,professors:Professor,candidates:Candidate];
I run the application with no problems, but when I used show tables the transaction tables weren't all created. It just created the last parameter's table (candidate).
Any idea about the reason and how to fix it? everywhere I checked, people did the same as me and had no problems with the tables. I'm using grails 2.4.4 version.
I don't think that your current domain has many-to-many relationship with all entities that you mentioned. It might be a one-to-many relationship. Let's say your current domain is "DomainA". If only other domain like AvaliateRules,Professor has "DomainA" in hasMany then it makes it a many-to-many association.
In one-to-many association, only a column is added at the child level(many side) which contains a parentId to denote who the parent is.
Related
Consider the scenario between a Project, Task, and a User (ER diagram attached included):
A project can have multiple tasks and a task can be in multiple projects (thus the many-many relationship ProjectTask)
A project can have multiple users and a user can be in multiple projects (thus the many-many relationship ProjectUser)
Check the above example's ER Diagram here
Now a ProjectTask can be assigned to multiple ProjectUsers and a ProjectUser can be assigned multiple ProjectTasks, basically a TasksAssigned relationship
How will I go about storing that information in a database? Can two relationships have a relationship? If yes, is there a better way to showcase this scenario? And if no, how do you show this relationship?
You can have a three-way relationship, in your case:
user
project
task
You probably want to guarantee that the user and task have the same project. That can be handled by declaring composite foreign keys to the projectTasks and projectUsers tables. The structure guarantees that the user and task are in the same project (because it is explicitly mentioned in the row in this table).
I'm working with a big mySQL database in IntelliJ.
I'm trying to join two tables which aren't in a direct key-relation to each other. Hence I have to join over multiple tables.
Because I barley know the database scheme, I can't find out these tables in a appropriate time. I know that IntelliJ has a function which can visualize all tables with their relations within the database but does it also provide a function where I can find out all tables in between two specific tables?
You can get help from Intellij partially for your task, using the visualization feature as follows-
The relationship between tables are clearly shown.
For your question, you need to check the primary and foreign keys for each table which are easy to know as they are highlighted.
Traversing them you can find the relationship.
I have three tables: Resumes, Orgs, and Resume2Org. Basically, Resume2Org is my many-to-many relationship table linking Resumes.resume_id to Orgs.org_id (so it only has those two keys in that table).
My question is, is it okay to use that many-to-many relationship table to store other data? My use case: the database is part of a system to sift through incoming resumes. But I've been asked to implement a "marked as read" feature so we can easily get the list of resumes we haven't looked at yet. But since a resume can belong to many different orgs, we only want to mark a resume as read for the org the user/viewer belongs to. I thought, hey, having that flag in Resume2Org would be perfect. Is this a smart approach, or should I create a new table specifically for "marked as read"? All the examples I've seen about many-to-many relationship tables is that those tables are used just for that... linking two tables.
Yes it is okey to have additional fields in a many-to-many table. I think it is the right way to do in your case as you don't need to join additional tables and you save spaces.
I was in a very similar situation last week and I added additional field for that.
I am using ASP.NET MVC2 on top of a MySQL database in VS2008. I am using the MySQL ADO.NET connector 6.2.3 to provide the connection for the ADO.NET Entity Data Model.
This is mostly working ok, however navigating via foreign keys is causing me a real headache!
Here is a simplified example..
Car (Table)
CarID PK
Colour
Doors
ManufacturerID FK
Manufacturer (Table)
ManufacturerID PK
Name
In the edmx file I can see the 1-many relationship shown as a navigation property in the both the Car and Manufacturer tables. I create a Models.CarRepository that allows me to returns a IQueryable.
At the View I want to be able to display the Manufacturer.Name for each car. This is not accessible via the object I get returned.
What is best way to implement this? Have I encountered a limitation of the Entity Framework/MySQL combination?
Eager loading of the related records needs to be enabled in the Model Repository. Something like:
var allCars = from c in automobileEntites.Car.Include("Manufacturer")
select c;
This then makes the related records available for subsequent query/display.
I'm playing around with LinqToSQL using an existing multi-lingual database, but I'm running into issues mapping a fairly important one-to-one relationship, so I suspect I am using the feature incorrectly for my database design.
Assume two tables, Category and CategoryDetail. Category contains the CategoryId (PK), ParentId and TemplateId. CategoryDetail contains the CategoryId (FK), LanguageId, Title and Description (in the appropriate language), with a combined PK of CategoryId and LanguageId.
If I drag-and-drop these tables into the LinqToSQL designer, the resultant object model has Category with a collection of CategoryDetail objects, which should never be the case. I'd like to be able to filter on LanguageId at the DataContext level, meaning that the whole Category is encapsulated within Category.CategoryDetail, not all language version encapsulated within Category.CategoryDetails.
This database worked fine on my old object library (an old-school custom BOL and DAL), but I fear that LinqToSQL would require this to change in order to give me the required result.
What is the best way to make this relationship (and language filtering) as seamless as possible?
You can view properties of the association. (Right click on the line representing the association and show properties.) The properties will tell you if it is a one-to-one or one-to-many relationship. This is reflected in code by having either a single entity association (one-to-one) or an entity set association (one-to-many).
I would have to assume cant be a true 1 to 1. Sounds like you have a PK of CatID and Lang ID on the Cat Details table. That would explain why its putting a collection. I could be wrong as you didnt mention the PK's of the CatDetails table
EDIT: A combined Pk of CatID and Lang ID makes that a 1:m relationship, and Linq to SQL is actually doing the correct thing. The only way it could possibly be a true 1:1 is if you had a lang ID on the cat table as well and that was part of the FK. I htink you may have to rethink what you want to do, or how you want to implement it.
I think LINQ to SQL models the database structure directly.
You have two tables so it creates 2 objects.
Have you had a look at LINQ to Entities this allows you to create another layer above the database strucure to make for more readable classes.
Since you don't have a 1:1 relationship the mapping alone will not provide the desired functionality. However it is easy to provide a method in the parent auto-generated class that does the job:
public partial class Category
{
public IEnumerable<CategoryDetail> GetDetailsByLanguage(string langID)
{
return this.CategoryDetails.Where(c => c.LangID == langID);
}
}