I am doing a Rails 3 app that replaces a paper form for a company. The paper form spans two pages and contains a LOT of fields, checkboxes, drop downs, etc.
I am wondering how to model that in the DB - one approach is to just create a field in the DB for every field on the form (normalized of course). That will make it somewhat difficult to ad or remove fileds since a migration will be needed. An other approach is to do some kind of key/value store (no - MongoDB/CouchDB is not an option - MySQL is required). Doing key/value will be very flexible but will be a pain to query. And it will directly work against ActiveRecord?
Anyone have a great solution for this?
Regards,
Jacob
I would recommend that you model the most common attributes as separate database fields. Once you have setup as many fields as possible then fall back to using a key-value setup for your pseudo-random attributes. I'd recommend a simple approach of storing a Hash through the ActiveRecord method serialize. For example:
class TPS < ActiveRecord::Base
serialize :custom, Hash
end
#tps = TPS.create(:name => "Kevin", :ssn => "123-456-789", :custom => { :abc => 'ABC', :def => )'DEF' })
#tps.name # Kevin
#tps.ssn # 123-456-789
#tps.custom[:abc] # ABC
#tps.custom[:def] # DEF
If your form is fairly static, go ahead and make a model for it, that's a reasonable approach even if it seems rather rudimentary. It's not your fault the form is so complicated, you're just coming up with a solution that takes that into account. Migrations to make adjustments to this are really simple to implement and easy to understand.
Splitting it up into a key/value version would be better but would take a lot more engineering. If you expect that this form will be subject to frequent and radical revisions it may make more sense to build for the future in this regard. You can see an example of the sort of form-builder you might want to construct at something like WuFoo but of course building form builders is not to be taken lightly.
Related
I am try to utilize JSON data to dynamically generate a form flow. In the Improved Sandwich Bot, each field in the form flow is independent to each other. For example, no matter I choose what kind of sandwich, I can continue to choose any type of bread. The only way to add some customization is using the following code:
.Field(new FieldJson(schema, "Specials")
.SetType(null)
.SetActive((state) => (string)state["Length"] == "FootLong")
.SetDefine(async (state, field) =>
{
field
.AddDescription("cookie", "FreeCookie")
.AddTerms("cookie", "cookie", "FreeCookie")
.AddDescription("drink", "FreeDrink")
.AddTerms("drink", "drink", "FreeDrink");
return true;
}))
However, since different sandwich stores have different menus, the dependency between different fields varies a lot. For example,
Store A may say only Sandwich1 can have toppings1, 2, 3. And store B
may say only Bread1 can have cheese1, 2, 3.
So I don't want to use the code above to implement the logic. It is not scalable.
So is it possible to include those dependency relations in the JSON file? In that way, the form builder can directly build the form flow with certain dependency relation.
No, it's not possible at this point but it seems like a very good suggestion. You can give the feedback at https://feedback.botframework.com/.
It is my first time to use yii and unlike my old programming style, i notice that it use relationship automatically in its model.
public function relations()
{
return array(
'author'=>array(self::BELONGS_TO, 'User', 'author_id'),
'categories'=>array(self::MANY_MANY, 'Category',
'tbl_post_category(post_id, category_id)'),
);
}
I'm not used in doing this MySQL relationship. my old programming habit is connecting/manipulating the data to the php program itself.. To clarify my question, is this yii model relationship important? if i dont use this method, will i encounter problems?
Yii relations are very useful and if you work with it you will see that it will make you do less coding and make your code more readable.
while it is so much used in Yii applications, if you don't use relations, you won't get into any trouble, it is supposed to help you code and develop faster.
like if you looked at Yii blog, you have relation between Post model and Comments model, and you could go like this:
$post = Post::model()->findByPk( $id ); // find one post
$allCommentsRelated = $post->comments; // just one line for all search query and instanciating models
BTW in relations, there are two type of loading:
lazy loading (this is default mechanism)
eager loading
you have to know your scenario, and choose one that suites that scenario best
I read quite a lot about the visitor pattern and its supposed advantages. To me however it seems they are not that much advantages when applied in practice:
"Convenient" and "elegant" seems to mean lots and lots of boilerplate code
Therefore, the code is hard to follow. Also 'accept'/'visit' is not very descriptive
Even uglier boilerplate code if your programming language has no method overloading (i.e. Vala)
You cannot in general add new operations to an existing type hierarchy without modification of all classes, since you need new 'accept'/'visit' methods everywhere as soon as you need an operation with different parameters and/or return value (changes to classes all over the place is one thing this design pattern was supposed to avoid!?)
Adding a new type to the type hierarchy requires changes to all visitors. Also, your visitors cannot simply ignore a type - you need to create an empty visit method (boilerplate again)
It all just seems to be an awful lot of work when all you want to do is actually:
// Pseudocode
int SomeOperation(ISomeAbstractThing obj) {
switch (type of obj) {
case Foo: // do Foo-specific stuff here
case Bar: // do Bar-specific stuff here
case Baz: // do Baz-specific stuff here
default: return 0; // do some sensible default if type unknown or if we don't care
}
}
The only real advantage I see (which btw i haven't seen mentioned anywhere): The visitor pattern is probably the fastest method to implement the above code snippet in terms of cpu time (if you don't have a language with double dispatch or efficient type comparison in the fashion of the pseudocode above).
Questions:
So, what advantages of the visitor pattern have I missed?
What alternative concepts/data structures could be used to make the above fictional code sample run equally fast?
For as far as I have seen so far there are two uses / benefits for the visitor design pattern:
Double dispatch
Separate data structures from the operations on them
Double dispatch
Let's say you have a Vehicle class and a VehicleWasher class. The VehicleWasher has a Wash(Vehicle) method:
VehicleWasher
Wash(Vehicle)
Vehicle
Additionally we also have specific vehicles like a car and in the future we'll also have other specific vehicles. For this we have a Car class but also a specific CarWasher class that has an operation specific to washing cars (pseudo code):
CarWasher : VehicleWasher
Wash(Car)
Car : Vehicle
Then consider the following client code to wash a specific vehicle (notice that x and washer are declared using their base type because the instances might be dynamically created based on user input or external configuration values; in this example they are simply created with a new operator though):
Vehicle x = new Car();
VehicleWasher washer = new CarWasher();
washer.Wash(x);
Many languages use single dispatch to call the appropriate function. Single dispatch means that during runtime only a single value is taken into account when determining which method to call. Therefore only the actual type of washer we'll be considered. The actual type of x isn't taken into account. The last line of code will therefore invoke CarWasher.Wash(Vehicle) and NOT CarWasher.Wash(Car).
If you use a language that does not support multiple dispatch and you do need it (I can honoustly say I have never encountered such a situation though) then you can use the visitor design pattern to enable this. For this two things need to be done. First of all add an Accept method to the Vehicle class (the visitee) that accepts a VehicleWasher as a visitor and then call its operation Wash:
Accept(VehicleWasher washer)
washer.Wash(this);
The second thing is to modify the calling code and replace the washer.Wash(x); line with the following:
x.Accept(washer);
Now for the call to the Accept method the actual type of x is considered (and only that of x since we are assuming to be using a single dispatch language). In the implementation of the Accept method the Wash method is called on the washer object (the visitor). For this the actual type of the washer is considered and this will invoke CarWasher.Wash(Car). By combining two single dispatches a double dispatch is implemented.
Now to eleborate on your remark of the terms like Accept and Visit and Visitor being very unspecific. That is absolutely true. But it is for a reason.
Consider the requirement in this example to implement a new class that is able to repair vehicles: a VehicleRepairer. This class can only be used as a visitor in this example if it would inherit from VehicleWasher and have its repair logic inside a Wash method. But that ofcourse doesn't make any sense and would be confusing. So I totally agree that design patterns tend to have very vague and unspecific naming but it does make them applicable to many situations. The more specific your naming is, the more restrictive it can be.
Your switch statement only considers one type which is actually a manual way of single dispatch. Applying the visitor design pattern in the above way will provide double dispatch.
This way you do not necessarily need additional Visit methods when adding additional types to your hierarchy. Ofcourse it does add some complexity as it makes the code less readable. But ofcourse all patterns come at a price.
Ofcourse this pattern cannot always be used. If you expect lots of complex operations with multiple parameters then this will not be a good option.
An alternative is to use a language that does support multiple dispatch. For instance .NET did not support it until version 4.0 which introduced the dynamic keyword. Then in C# you can do the following:
washer.Wash((dynamic)x);
Because x is then converted to a dynamic type its actual type will be considered for the dispatch and so both x and washer will be used to select the correct method so that CarWasher.Wash(Car) will be called (making the code work correctly and staying intuitive).
Separate data structures and operations
The other benefit and requirement is that it can separate the data structures from the operations. This can be an advantage because it allows new visitors to be added that have there own operations while it also allows data structures to be added that 'inherit' these operations. It can however be only applied if this seperation can be done / makes sense. The classes that perform the operations (the visitors) do not know the structure of the data structures nor do they have to know that which makes code more maintainable and reusable. When applied for this reason the visitors have operations for the different elements in the data structures.
Say you have different data structures and they all consist of elements of class Item. The structures can be lists, stacks, trees, queues etc.
You can then implement visitors that in this case will have the following method:
Visit(Item)
The data structures need to accept visitors and then call the Visit method for each Item.
This way you can implement all kinds of visitors and you can still add new data structures as long as they consist of elements of type Item.
For more specific data structures with additional elements (e.g. a Node) you might consider a specific visitor (NodeVisitor) that inherits from your conventional Visitor and have your new data structures accept that visitor (Accept(NodeVisitor)). The new visitors can be used for the new data structures but also for the old data structures due to inheritence and so you do not need to modify your existing 'interface' (the super class in this case).
In my personal opinion, the visitor pattern is only useful if the interface you want implemented is rather static and doesn't change a lot, while you want to give anyone a chance to implement their own functionality.
Note that you can avoid changing everything every time you add a new method by creating a new interface instead of modifying the old one - then you just have to have some logic handling the case when the visitor doesn't implement all the interfaces.
Basically, the benefit is that it allows you to choose the correct method to call at runtime, rather than at compile time - and the available methods are actually extensible.
For more info, have a look at this article - http://rgomes-info.blogspot.co.uk/2013/01/a-better-implementation-of-visitor.html
By experience, I would say that "Adding a new type to the type hierarchy requires changes to all visitors" is an advantage. Because it definitely forces you to consider the new type added in ALL places where you did some type-specific stuff. It prevents you from forgetting one....
This is an old question but i would like to answer.
The visitor pattern is useful mostly when you have a composite pattern in place in which you build a tree of objects and such tree arrangement is unpredictable.
Type checking may be one thing that a visitor can do, but say you want to build an expression based on a tree that can vary its form according to a user input or something like that, a visitor would be an effective way for you to validate the tree, or build a complex object according to the items found on the tree.
The visitor may also carry an object that does something on each node it may find on that tree. this visitor may be a composite itself chaining lots of operations on each node, or it can carry a mediator object to mediate operations or dispatch events on each node.
You imagination is the limit of all this. you can filter a collection, build an abstract syntax tree out of an complete tree, parse a string, validate a collection of things, etc.
I'm writing a web application (in Python, not that it matters). One of the features is that people can leave comments on things. I have a class for comments, basically like so:
class Comment:
user = ...
# other stuff
where user is an instance of another class,
class User:
name = ...
# other stuff
And of course in my template, I have
<div>${comment.user.name}</div>
Problem: Let's say I allow people to post comments anonymously. In that case comment.user is None (undefined), and of course accessing comment.user.name is going to raise an error. What's the best way to deal with that? I see three possibilities:
Use a conditional in the template to test for that case and display something different. This is the most versatile solution, since I can change the way anonymous comments are displayed to, say, "Posted anonymously" (instead of "Posted by ..."), but I've often been told that templates should be mindless display machines and not include logic like that. Also, other people might wind up writing alternate templates for the same application, and I feel like I should be making things as easy as possible for the template writer.
Implement an accessor method for the user property of a Comment that returns a dummy user object when the real user is undefined. This dummy object would have user.name = 'Anonymous' or something like that and so the template could access it and print its name with no error.
Put an actual record in my database corresponding to a user with user.name = Anonymous (or something like that), and just assign that user to any comment posted when nobody's logged in. I know I've seen some real-world systems that operate this way. (phpBB?)
Is there a prevailing wisdom among people who write these sorts of systems about which of these (or some other solution) is the best? Any pitfalls I should watch out for if I go one way vs. another? Whoever gives the best explanation gets the checkmark.
I'd go with the first option, using an if switch in the template.
Consider the case of localization: You'll possibly have different templates for each language. You can easily localize the "anonymous" case in the template itself.
Also, the data model should have nothing to do with the output side. What would you do in the rest of the code if you wanted to test whether a user has a name or not? Check for == 'Anonymous' each time?
The template should indeed only be concerned with outputting data, but that doesn't mean it has to consist solely of output statements. You usually have some sort of if user is logged in, display "Logout", otherwise display "Register" and "Login" case in the templates. It's almost impossible to avoid these.
Personally, I like for clean code, and agree that templates should not have major logic. So in my implementations I make sure that all values have "safe" default values, typically a blank string, pointer to a base class or equivalent. That allows for two major improvements to the code, first that you don't have to constantly test for null or missing values, and you can output default values without too much logic in your display templates.
So in your situation, making a default pointer to a base value sounds like the best solution.
Your 3rd option: Create a regular User entity that represents an anonymous user.
I'm not a fan of None for database integrity reasons.
I want to create a good syntax for making it easier to link to internal things, kind of like a Wiki, so maybe I'm thinking the user can use something like:
Bah bah bah reference: [[a]]
And it would convert it into HTML:
Bah bah bah reference: title
but I'm worry about interjection and security issues. What's a good syntax to pick (maybe [[]] or something else) and what's a safe regex for it?
Don't reinvent the wheel: Text::Markdown.
While Sinan's idea of using an existing syntax, such as Markdown, is a good one, the viability of this sort of approach depends on the particulars of your situation. You haven't told us enough to get very helpful responses. Are you starting from a blank slate or are you inheriting a large body of existing documents that would require a costly conversion process, not to mention staff retraining?
If the latter, sometimes it's best to tackle the problem by thinking in terms of short-term workarounds and long-term strategies. The short-term workaround -- some sort of home-grown syntax precisely like that proposed in your question -- will be ugly and may cause hassles here and there, but it can get the job done. For example, our organization has a large pile of Word documents that ultimately provide content for HTML pages. Within those Word files, staff member use an approach like this to create links and make a few other declarations that our parsing code handles: ##some_link##. There are many ways for this to fail, but in the type of content that we are producing, it rarely occurs. Partly for that reason, it's difficult to generate much enthusiasm for a long-term strategy of migrating the content to a more robust system. My expectation is that such a migration will occur, but it is being driven by larger considerations and not by the limitations of our kludgy ##foo## markup device.
Update:
Based on your additional comments, it sounds like you will have a list of links that you want your users to be able to add quickly, using short IDs. So, those links will be defined someplace, say a Perl hash. You can use this hash to guard against invalid entries. For example:
use strict;
use warnings;
my %approved_links = (
so_perl => 'http://stackoverflow.com/questions/tagged/perl',
so_ruby => 'http://stackoverflow.com/questions/tagged/ruby',
);
my $regex = qr/ \[\[ ([^\]]+) \]\] /x;
while (<DATA>){
die "Danger: $_\n" if map {
exists $approved_links{$_} ? () : ($_)
} /$regex/g;
s/$regex/$approved_links{$1}/g; # Not complete, but you get the idea.
print;
}
__DATA__
Some text [not an id] [[so_perl]] more [[so_ruby]] more..
[[so_perl]] blah [[so_ruby]] blah
[[bad id!!]]
Something like Textile or Creole? Why don't you use an existing implementation?