I am trying to create an intermediate model between two models that I'd like to have a many to many relationship. I am creating an atypical book checkout project and have two models setup Book and Person. I'd like to setup an intermediate model BookCheckOut to track OutDate and ReturnDate.
Dan Singerman provided what looks like the answer I am looking for to the question Ruby On Rails Relationships - One to Many. My inexperience with out model generation and my reliance upon scaffolding are probably causing my problem. I am trying to determine how to not only generate the model but a working database migration that would accompany it.
I am not a rails maestro, but there are two ways to do it that I know of: has_many :through and has_and_belongs_to_many. This article has a decent overview of how. I suspect you would want to use has_many :through so you can access the data in the join table cleanly.
To generate the intermediate model you would do something like:
script/generate model checkouts person_id:int, book_id:int, checked_out:date, returned:date
In your Book model you would add (does Rails know Person --> "People"? I'm guessing yes):
has_many :people, :through => :checkouts
has_many :checkouts, :dependent => true
In your Person model you would add:
has_many :books, :through => :checkouts
has_many :checkouts, :dependent => true
In your Checkout (sorry, I renamed it from your example) model, you would add:
belongs_to :person
belongs_to :book
Use caution with my examples - I am going from memory.
Related
I'm designing an application to create goals for a classroom - at the moment, the data relationships are modeled like this
I knew that out of inexperience I might run into problems and now I'm beginning to - specifically in my routes file - but also just for generally organizing the site and the data relationships.
When a user signs up, they add a student_group (or class), populate it with students and then add subjects. Later they add goals for each subject - although there should also be goals for a student_group, a student, or even the user. I was thinking of something like this - but would it be better as a has_many, through relationship?
Right now, I've only really done work on the User, Student_group, and Student models and these are fairly straight-forward. A user has many student_groups, and a student_group has many students. I'd like a second opinion before I proceed however, so that I don't have to end up going back and doing things over. Thanks!
I think you might be thinking too far ahead. Once you have your app built around your current data model, you'll know better whether you even want to expand it to include the concept of a goal that isn't part of a student's subject. If you decide that it is, then making goals belong_to a subject, student, or user will be pretty simple. At that point, you could also do something like
Class Student
has_many :personal_goals, class_name: "Goal"
has_many :goals, through: :subjects
def all_goals
goals + personal_goals
end
There's probably a more elegant way to model that last relationship. Would you need to go beyond that? Does it make sense to talk about a student group having a goal of its own? I don't know.
As I gone through your database design, I have found that you should have to use different type of relationships, that rails has provided us. I tried my best to design your schema as per my knowledge. you should define relationship in your model as I suggested below. Any good modification are highly appreciated.
User
has_many :student_groups
has_many :students, through: :student_groups
has_many :goals, as: :goalable
StudentGroup
belongs_to :user
has_many :students
has_many :goals, as: :goalable
Student
belongs_to :student_group
has_many :subjects
has_many :characteristics
has_many :goals, as: :goalable
Characteristic
belongs_to :student
Goal
belongs_to :goalable, polymorphic => true
I have defined some polymorphic associations in your schema. If you need any reference related to these association. visit http://guides.rubyonrails.org/association_basics.html
Hope it will help you. Thanks.
So, I'm working on an app where I want to users to be able to group objects in "folders". Basically:
User has_many :foos
Foos don't have to be in a folder, but they can be. In that case:
Folder has_many :foos and Foo belongs_to :folder
Now, I'd like to be able to set up folders so they can be nested. I think this is something like...
Folder has_many :folders
I have heard that this kind of self-referential relationship is no big deal, but I don't really get how it works. I haven't been able to figure out how this is supposed to be declared in the model and what columns I need to provide in the database.
Could anyone offer an example? I'd also value any suggestions/heads-up/warnings/lessons learned that you might be able to offer about setting up this kind of relationship in an app.
Thanks!
Checkout coreyward's answer to the question here: Creating a model that has a tree structure
Basically you want to add a "parent_id" field to your folders table and then set up a relationship in your Folder model like this:
belongs_to :parent, :class_name => "Folder"
has_many :folders, :foreign_key => "parent_id"
See comments for updates.
I've been struggling to get a clear and straight-forward answer on this one, I'm hoping this time I'll get it! :D
I definitely have a lot to learn still with Rails, however I do understand the problem I'm facing and would really appreciate additional help.
I have a model called "Task".
I have an abstract model called "Target".
I would like to relate multiple instances of subclasses of Target to Task.
I am not using single table inheritance.
I would like to query the polymorphic relationship to return a mixed result set of subclasses of Target.
I would like to query individual instances of subclasses of Target to obtain tasks that they are in a relationship with.
So, I figure a polymorphic many to many relationship between Tasks and subclasses of Targets is in order.
In more detail, I will be able to do things like this in the console (and of course elsewhere):
task = Task.find(1)
task.targets
[...array of all the subclasses of Target here...]
But! Assuming models "Store", "Software", "Office", "Vehicle", which are all subclasses of "Target" exist, it would be nice to also traverse the relationship in the other direction:
store = Store.find(1)
store.tasks
[...array of all the Tasks this Store is related to...]
software = Software.find(18)
software.tasks
[...array of all the Tasks this Software is related to...]
The database tables implied by polymorphic relationships appears to be capable of doing this traversal, but I see some recurring themes in trying to find an answer which to me defeat the spirit of polymorphic relationships:
Using my example still, people appear to want to define Store, Software, Office, Vehicle in Task, which we can tell right away isn't a polymorphic relationship as it only returns one type of model.
Similar to the last point, people still want to define Store, Software, Office and Vehicle in Task in one way shape or form. The important bit here is that the relationship is blind to the subclassing. My polymorphs will initially only be interacted with as Targets, not as their individual subclass types. Defining each subclass in Task again starts to eat away at the purpose of the polymorphic relationship.
I see that a model for the join table might be in order, that seems somewhat correct to me except that it adds some complexity I assumed Rails would be willing to do away with. I plea inexperience on this one.
It seems to be a small hole in either rails functionality or the collective community knowledge. So hopefully stackoverflow can chronicle my search for the answer!
Thanks to everyone who help!
You can combine polymorphism and has_many :through to get a flexible mapping:
class Assignment < ActiveRecord::Base
belongs_to :task
belongs_to :target, :polymorphic => true
end
class Task < ActiveRecord::Base
has_many :targets, :through => :assignment
end
class Store < ActiveRecord::Base
has_many :tasks, :through => :assignment, :as => :target
end
class Vehicle < ActiveRecord::Base
has_many :tasks, :through => :assignment, :as => :target
end
...And so forth.
Although the answer proposed by by SFEley is great, there a some flaws:
The retrieval of tasks from target (Store/Vehicle) works, but the backwards wont. That is basically because you can't traverse a :through association to a polymorphic data type because the SQL can't tell what table it's in.
Every model with a :through association need a direct association with the intermediate table
The :through Assignment association should be in plural
The :as statement wont work together with :through, you need to specify it first with the direct association needed with the intermediate table
With that in mind, my simplest solution would be:
class Assignment < ActiveRecord::Base
belongs_to :task
belongs_to :target, :polymorphic => true
end
class Task < ActiveRecord::Base
has_many :assignments
# acts as the the 'has_many targets' needed
def targets
assignments.map {|x| x.target}
end
end
class Store < ActiveRecord::Base
has_many :assignments, as: :target
has_many :tasks, :through => :assignment
end
class Vehicle < ActiveRecord::Base
has_many :assignments, as: :target
has_many :tasks, :through => :assignment, :as => :target
end
References:
http://blog.hasmanythrough.com/2006/4/3/polymorphic-through
The has_many_polymorphs solution you mention isn't that bad.
class Task < ActiveRecord::Base
has_many_polymorphs :targets, :from => [:store, :software, :office, :vehicle]
end
Seems to do everything you want.
It provides the following methods:
to Task:
t = Task.first
t.targets # Mixed collection of all targets associated with task t
t.stores # Collection of stores associated with task t
t.softwares # same but for software
t.offices # same but for office
t.vehicles # same but for vehicles
to Software, Store, Office, Vehicle:
s = Software.first # works for any of the subtargets.
s.tasks # lists tasks associated with s
If I'm following the comments correctly, the only remaining problem is that you don't want to have to modify app/models/task.rb every time you create a new type of Subtarget. The Rails way seems to require you to modify two files to create a bidirectional association. has_many_polymorphs only requires you to change the Tasks file. Seems like a win to me. Or at least it would if you didn't have to edit the new Model file anyway.
There are a few ways around this, but they seem like way too much work to avoid changing one file every once in a while. But if you're that dead set against modifying Task yourself to add to the polymorphic relationship, here's my suggestion:
Keep a list of subtargets, I'm going to suggest in lib/subtargets formatted one entry per line that is essentially the table_name.underscore. (Capital letters have an underscore prefixed and then everything is made lowercase)
store
software
office
vehicle
Create config/initializers/subtargets.rb and fill it with this:
SubtargetList = File.open("#{RAILS_ROOT}/lib/subtargets").read.split.reject(&:match(/#/)).map(&:to_sym)
Next you're going to want to either create a custom generator or a new rake task. To generate your new subtarget and add the model name to the subtarget list file, defined above. You'll probably end up doing something bare bones that makes the change and passes the arguments to the standard generator.
Sorry, I don't really feel like walking you through that right now, but here are some resources
Finally replace the list in the has_many_polymorphs declaration with SubtargetList
class Task < ActiveRecord::Base
has_many_polymorphs :targets, :from => SubtargetList
end
From this point on you could add a new subtarget with
$ script/generate subtarget_model home
And this will automatically update your polymorphic list once you reload your console or restart the production server.
As I said it's a lot of work to automatically update the subtargets list. However, if you do go this route you can tweak the custom generator ensure all the required parts of the subtarget model are there when you generate it.
Using STI:
class Task < ActiveRecord::Base
end
class StoreTask < Task
belongs_to :store, :foreign_key => "target_id"
end
class VehicleTask < Task
belongs_to :vehicle, :foreign_key => "target_id"
end
class Store < ActiveRecord::Base
has_many :tasks, :class_name => "StoreTask", :foreign_key => "target_id"
end
class Vehicle < ActiveRecord::Base
has_many :tasks, :class_name => "VehicleTask", :foreign_key => "target_id"
end
In your databse you'll need:
Task type:string and Task target_id:integer
The advantage is that now you have a through model for each task type which can be specific.
See also STI and polymorphic model together
Cheers!
This may not be an especially helpful answer, but stated simply, I don't think there is an easy or automagic way to do this. At least, not as easy as with simpler to-one or to-many associations.
I think that creating an ActiveRecord model for the join table is the right way to approach the problem. A normal has_and_belongs_to_many relationship assumes a join between two specified tables, whereas in your case it sounds like you want to join between tasks and any one of stores, softwares, offices, or vehicles (by the way, is there a reason not to use STI here? It seems like it would help reduce complexity by limiting the number of tables you have). So in your case, the join table would also need to know the name of the Target subclass involved. Something like
create_table :targets_tasks do |t|
t.integer :target_id
t.string :target_type
t.integer :task_id
end
Then, in your Task class, your Target subclasses, and the TargetsTask class, you could set up has_many associations using the :through keyword as documented on the ActiveRecord::Associations::ClassMethods rdoc pages.
But still, that only gets you part of the way, because :through won't know to use the target_type field as the Target subclass name. For that, you might be able to write some custom select/finder SQL fragments, also documented in ActiveRecord::Associations::ClassMethods.
Hopefully this gets you moving in the right direction. If you find a complete solution, I'd love to see it!
I agree with the others I would go for a solution that uses a mixture of STI and delegation would be much easier to implement.
At the heart of your problem is where to store a record of all the subclasses of Target. ActiveRecord chooses the database via the STI model.
You could store them in a class variable in the Target and use the inherited callback to add new ones to it. Then you can dynamically generate the code you'll need from the contents of that array and leverage method_missing.
Have you pursued that brute force approach:
class Task
has_many :stores
has_many :softwares
has_many :offices
has_many :vehicles
def targets
stores + softwares + offices + vehicles
end
...
It may not be that elegant, but to be honest it's not that verbose, and there is nothing inherently inefficient about the code.
Rails has a has_one :through association that helps set up a one-to-one association with a third model by going through a second model. What is the real use of that besides making a shortcut association, that would otherwise be an extra step away.
Taking this example from the Rails guide:
class Supplier < ActiveRecord::Base
has_one :account
has_one :account_history, :through => :account
end
class Account < ActiveRecord::Base
belongs_to :supplier
has_one :account_history
end
class AccountHistory < ActiveRecord::Base
belongs_to :account
end
might allow us to do something like:
supplier.account_history
which would otherwise be reached as:
supplier.account.history
If it's only for simpler access then technically there could be a one-to-one association that connects a model with some nth model going through n-1 models for easier access. Is there anything else to it that I am missing besides the shortcut?
Logic, OK it might sound a bit weak for this but it would be logical to say that "I have a supplier who has an account with me, I want to see the entire account history of this supplier", so it makes sense for me to be able to access account history from supplier directly.
Efficiency, this for me is the main reason I would use :through, simply because this issues a join statement rather than calling supplier, and then account, and then account_history. noticed the number of database calls?
using :through, 1 call to get the supplier, 1 call to get account_history (rails automatically uses :join to retrieve through account)
using normal association, 1 call to get supplier, 1 call to get account, and 1 call to get account_history
That's what I think =) hope it helps!
I'm surprised no one has touched on Association Objects.
A has_many (or has_one) :through relationship facilitates the use of the association object pattern which is when you have two things related to each other, and that relation itself has attributes (ie a date when the association was made or when it expires).
This is considered by some to be a good alternative to the has_and_belongs_to_many ActiveRecord helper. The reasoning behind this is that it is very likely that you will need to change the nature of the association or add to it, and when you are a couple months into a project, this can be very painful if the relationship were initially set up as a has_and_belongs_to_many (the second link goes into some detail). If it is set up initially using a has_many :through relationship, then a couple months into the project it's easy to rename the join model or add attributes to it, making it easier for devs to respond to changing requirements. Plan for change.
Inverse association: consider the classic situation user-membership-group. If a user can be a member in many groups, then a group has many members or users, and a user has many groups. But if the user can only be a member in one group, the group still has many members: class User has_one :group, :through => :membership but class Group has_many :members, :through => memberships. The intermediate model membership is useful to keep track of the inverse relationship.
Expandability: a has_one :through relationship can easy be expanded and extended to a has_many :through relationship
Sorry for the vague title, but this is kind of hard to put into one line.
I have a database full of contact information, and I want to be able to put those different contacts into groups which will also be stored in the database.
So, maybe I have 2 groups, "coworkers" and "neighbors", I want to be able to see a list of all my contacts and be able to add individual contacts to one or more groups.
I'm kind of confused as where to begin with this, could I get some basic outlines of how this might best be implemented? Thanks.
Well, you've got two models, Contact and Group. These two guys are clearly going to have their own tables (probably 'contacts' and 'groups' if you're following the Rails conventions). Since a contact can be in many groups, and a group can have many contacts, you've got what's called a many-to-many relationship between these models. There are basically two ways to implement this in Rails: has_and_belongs_to_many or has_many :through. Which one is best for you will depend a little on your situation.
has_and_belongs_to_many is probably the path of least resistance. You put a couple lines in your models like so:
# contact.rb
has_and_belongs_to_many :groups
# group.rb
has_and_belongs_to_many :contacts
...and create a table called 'contacts_groups' with the columns contact_id and group_id, and you're basically good to go. Easy peasy.
On the other hand, there are some advantages to using an association model with has_many :through. In this approach, you create another model, say, GroupMembership, and set up your models like so:
# contact.rb
has_many :group_memberships # this isn't strictly required, but I'd recommend it
has_many :groups, :through => :group_memberships
# group_membership.rb
has_many :groups
has_many :contacts
# group.rb
has_many :group_memberships # again, recommended but not required
has_many :contacts, :through => :group_memberships
This gives you most of the same convenience methods as has_and_belongs_to_many, and also lets you store any extra data you might want about the association, like the date they joined the group or the reason they were added. Even if you don't have any of that now, it's nice to account for the possibility of adding it later. Also, this lets you take a more RESTful approach to adding and removing contacts to and from groups, if that's something you're interested in, since you can model it in terms of creating and destroying GroupMembership resources.
By and large, I tend to lean toward the latter approach, especially as I've gotten more in to RESTful architectures. On the other hand, if you're not worried about REST and you're certain you'll never want to keep any extra information about memberships, has_and_belongs_to_many is probably simpler and requires less code to get working. For more in-depth information on the differences and implementation details, see the ActiveRecord Associations API docs.
You could structure a Ruby on Rails database to be :
Contact ( first_name:string, last_name:string, title:enum number:string, cell:string, notes:text, email:string ) => many_many :groups (or has_many :groups, :through=> :contact_group)
Contact_Group { group_id:integer, contact_id:integer }
Group ( name:string ) => many_many :contacts ) (or has_many :contacts, :through=> :contact_group)
That might be the general idea. You also could do relational fields.
You don't want to use the has_and_belongs_to_many approach. It's deprecated. Please read the API and make sure you implement a join model / has_many :through approach. The API will tell you how to do this and also mention why has_and_belongs_to_many is bad.
http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html