A Manager has many contacts via polymorphic association
class Manager
has_many :contacts, as: :contactable
end
class Contact
belongs_to :contactable, polymorphic: true
end
The relation works fine but now a contact can be associated to many managers.
So, added a new model Contactable, a joins table 'contactables' and moved contactable_id and contactable_type fields from contacts table to contactables table.
class Contactable
belongs_to :contact
belongs_to :contactable, polymorphic: true
end
Now confused about the Manager and Contact relation that how it would be defined in models correctly to make it working. Tried the following but it doesn't work:
class Manager
has_many :contacts, through: :contactables, source: :contactable, source_type: "Contact"
end
So I checked this interesting topic and will tell what I know.
When you create objects as usual in has_many :through:
class Contact
has_many :contactables
has_many :managers, :through => :contactables
end
class Manager
has_many :contactables
has_many :contacts, :through => :contactables
end
class Client
has_many :contactables
has_many :contacts, :through => :contactables
end
class Contactable
belongs_to :contact
belongs_to :manager
belongs_to :client
end
You get to use foreign keys fro each referenced object. Polymorphic looks like a great solution. So:
class Contactable
belongs_to :contact
belongs_to :polymorphic_model, polymorphic: true
end
class Contact
has_many :contactables
has_many :managers, :through => :contactables, :source => :polymorphic_model, :source_type => 'Manager'
end
class Manager
has_many :contactables, :as => :polymorphic_model
has_many :contacts, :through => :contactables
end
Setting the :as option indicates that this is a polymorphic
association
:source => :polymorphic_model is used to tell Rails to get the related object from the subclass. :source means the same as :class_name. Without this option Rails would try to get associated Manager from the Contactables table, while it should be reached via virtual Polymorphic_model.
By adding belongs_to :polymorphic_model to Contactable you enable Contact (witch already sits there, because of belongs_to :contact) to be associated with a Manager or Client, because thats what Polymorphic association does - references two or more parent tables. And because Contact have_many Contactables, the same Contact object can be associated with many managers or clients. So after you understand it, it looks really simple - Joined model belongs to Contact and Joined model also holds references to Manager and Client through Polymorphic association. So in order for Contact to have many managers, you create another Contactable object that belongs to the same Contact, but different Manager. Doesn't look super efficient, but personally me, not knowing a better way..
Here is a tested proof:
Manager.create!(name: "Bravo")
=> #<Manager id: 1, created_at: "2017-04-12 12:17:41", updated_at: "2017-04-12 12:17:41", name: "Bravo">
Manager.create!(name: "Johnny")
=> #<Manager id: 2, created_at: "2017-04-12 12:18:24", updated_at: "2017-04-12 12:18:24", name: "Johnny">
Contact.create!(number:"123")
=> #<Contact id: 1, created_at: "2017-04-12 12:18:59", updated_at: "2017-04-12 12:18:59", number: 123>
c = Contactable.new
c.contact = Contact.first
c.unit = Manager.first
c
=> #<Contactable id: nil, unit_type: "Manager", created_at: nil, updated_at: nil, unit_id: 1, contact_id: 1>
Now to set another Manager to the same contact, we create a new Contactable:
cc = Contactable.new
cc.contact = Contact.first
cc.unit = Manager.last
cc
=> #<Contactable id: nil, unit_type: "Manager", created_at: nil, updated_at: nil, unit_id: 4, contact_id: 1>
And to get all associated:
Contact.first.managers
Contactable's database:
contact_id
unit_id
unit_type
And one interesting quote by #Bill Karwin:
The Polymorphic Associations design breaks rules of relational
database design. I don't recommend using it.
But he wrote this long time ago. Probably irrelevant now.
Why can you not have a foreign key in a polymorphic association?
I'm running Rails 2.3.2 and doing:
class StandardWidget < ActiveRecord::Base
has_and_belongs_to_many :parts, :join_table => "widgets_parts", :association_foreign_key => "widget_custom_id"
end
class Part < ActiveRecord::Base
has_and_belongs_to_many :widgets, :join_table => "widgets_parts", :association_foreign_key => "part_custom_id"
end
p = StandardWidget.find(5)
p.widgets
and get the error
ActiveRecord::StatementInvalid: Mysql::Error: Unknown column 'widgets_parts.standard_widget_id' in 'where clause': SELECT * FROM `widgets` INNER JOIN `widgets_parts` ON `parts`.part_custom_id = `widgets_parts`.part_custom_id WHERE (`widgets_parts`.standard_widget_id = 5 )
How can I get this working?
The Rails documentation on HBTM says:
WARNING: If you‘re overwriting the
table name of either class, the
table_name method MUST be declared
underneath any has_and_belongs_to_many
declaration in order to work.
What does this mean?
You'll need to use :foreign_key also in the has_and_belongs_to_many call. So in the StandardWidget model you need this:
has_and_belongs_to_many :parts, :join_table => "widgets_parts", :association_foreign_key => "widget_custom_id", :foreign_key => "part_custom_id"
The warning in the docs means that if you are using table names other than 'parts' for the Part model and 'standard_widgets' for the StandardWidget model, then you need to call 'set_table_name' beneath the habtm call.
I have a Factsheet model which holds an assortment of publications, including alternate language versions. The different language versions should be kept separate as individual records (because they can be ordered/updated/etc. separately), but I'm trying to associate them to each other so that you can easily tell when one publication is the Spanish (or Chinese, etc.) version of the other.
I would like to use a :through association so that the relationship is symmetric, e.g. if English Factsheet A has a Spanish version Factsheet B, then similarly Factsheet B has an English version Factsheet A.
Here are my models:
class Factsheet < ActiveRecord::Base
has_many :publications_language_relationships
has_one :en, :through => :publications_language_relationships
has_one :es, :through => :publications_language_relationships
has_one :zh, :through => :publications_language_relationships #zh = Chinese
# other stuff
end
and...
# Table name: publications_language_relationships
#
# en_id :integer
# es_id :integer
# zh_id :integer
#
class PublicationsLanguageRelationship < ActiveRecord::Base
belongs_to :en, :class_name => 'Factsheet'
belongs_to :es, :class_name => 'Factsheet'
belongs_to :zh, :class_name => 'Factsheet'
end
But when I fire up a Rails console to check to see if that works at all...
$ fs = Factsheet.last
=> #<Factsheet id: 5, title: "Despu\xC3\xA9s de un diagn\xC3\xB3stico de c\xC3\
xA1ncer de seno: Con...", backend_code: "fs_after_bc_diagnosis_es", language:
"es", created_at: "2010-11-30 21:23:01", updated_at: "2010-12-06 16:13:23">
$ fs.en
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: publicati
ons_language_relationships.factsheet_id: SELECT "factsheets".* FROM "factsheets"
INNER JOIN "publications_language_relationships" ON "factsheets".id = "publicat
ions_language_relationships".en_id WHERE (("publications_language_relationships"
.factsheet_id = 5)) LIMIT 1
So something's amiss with my associations, but I'm not quite sure what. Thoughts?
Additionally, is this even a sound design for the data, or should I be doing something differently here?
I haven't tested this solution, but I think the general architectural direction you want to take is not really through the :through relationship, but rather something like:
class Factsheet < ActiveRecord::Base
has_many :publications_language_relationships
named_scope :translation, lambda { |trans|
{ :conditions => ["publications_language_relationships = ?", trans.to_s ] ,
:joins => :publications_language_relationships
}
}
# other stuff
end
class PublicationsLanguageRelationship < ActiveRecord::Base
belongs_to :fact_sheet
end
And then, I think you'd call the translations in your controller/views something like this:
#controller
def show
#fact_sheet = FactSheet.find( params[:id] ) # to load up the FactSheet
end
#view (to get the right translation)
#fact_sheet.translation(:en) #for english
I don't think this is absolutely right, but it should get you on the path anyway.
I've came into a problem while working with AR and polymorphic, here's the description,
class Base < ActiveRecord::Base; end
class Subscription < Base
set_table_name :subscriptions
has_many :posts, :as => :subscriptable
end
class Post < ActiveRecord::Base
belongs_to :subscriptable, :polymorphic => true
end
in the console,
>> s = Subscription.create(:name => 'test')
>> s.posts.create(:name => 'foo', :body => 'bar')
and it created a Post like:
#<Post id: 1, name: "foo", body: "bar", subscriptable_type: "Base", subscriptable_id: 1, created_at: "2010-05-10 12:30:10", updated_at: "2010-05-10 12:30:10">
the subscriptable_type is Base but Subscription, anybody can give me a hand on this?
If the class Base is an abstract model, you have to specify that in the model definition:
class Base < ActiveRecord::Base
self.abstract_class = true
end
Does your subscriptions table have a 'type' column? I'm guessing that Rails thinks that Base/Subscription are STI models. So when a row is retrieved from the subscriptions table and no 'type' column is present, it just defaults to the parent class of Base. Just a guess...
How can I make a many-to-many relationship with the same model in rails?
For example, each post is connected to many posts.
There are several kinds of many-to-many relationships; you have to ask yourself the following questions:
Do I want to store additional information with the association? (Additional fields in the join table.)
Do the associations need to be implicitly bi-directional?
(If post A is connected to post B, then post B is also connected to post A.)
That leaves four different possibilities. I'll walk over these below.
For reference: the Rails documentation on the subject. There's a section called “Many-to-many”, and of course the documentation on the class methods themselves.
Simplest scenario, uni-directional, no additional fields
This is the most compact in code.
I'll start out with this basic schema for your posts:
create_table "posts", :force => true do |t|
t.string "name", :null => false
end
For any many-to-many relationship, you need a join table. Here's the schema for that:
create_table "post_connections", :force => true, :id => false do |t|
t.integer "post_a_id", :null => false
t.integer "post_b_id", :null => false
end
By default, Rails will call this table a combination of the names of the two tables we're joining. But that would turn out as posts_posts in this situation, so I decided to take post_connections instead.
Very important here is :id => false, to omit the default id column. Rails wants that column everywhere except on join tables for has_and_belongs_to_many. It will complain loudly.
Finally, notice that the column names are non-standard as well (not post_id), to prevent conflict.
Now in your model, you simply need to tell Rails about these couple of non-standard things. It will look as follows:
class Post < ActiveRecord::Base
has_and_belongs_to_many(:posts,
:join_table => "post_connections",
:foreign_key => "post_a_id",
:association_foreign_key => "post_b_id")
end
And that should simply work! Here's an example irb session run through script/console:
>> a = Post.create :name => 'First post!'
=> #<Post id: 1, name: "First post!">
>> b = Post.create :name => 'Second post?'
=> #<Post id: 2, name: "Second post?">
>> c = Post.create :name => 'Definitely the third post.'
=> #<Post id: 3, name: "Definitely the third post.">
>> a.posts = [b, c]
=> [#<Post id: 2, name: "Second post?">, #<Post id: 3, name: "Definitely the third post.">]
>> b.posts
=> []
>> b.posts = [a]
=> [#<Post id: 1, name: "First post!">]
You'll find that assigning to the posts association will create records in the post_connections table as appropriate.
Some things to note:
You can see in the above irb session that the association is uni-directional, because after a.posts = [b, c], the output of b.posts does not include the first post.
Another thing you may have noticed is that there is no model PostConnection. You normally don't use models for a has_and_belongs_to_many association. For this reason, you won't be able to access any additional fields.
Uni-directional, with additional fields
Right, now... You've got a regular user who has today made a post on your site about how eels are delicious. This total stranger comes around to your site, signs up, and writes a scolding post on regular user's ineptitude. After all, eels are an endangered species!
So you'd like to make clear in your database that post B is a scolding rant on post A. To do that, you want to add a category field to the association.
What we need is no longer a has_and_belongs_to_many, but a combination of has_many, belongs_to, has_many ..., :through => ... and an extra model for the join table. This extra model is what gives us the power to add additional information to the association itself.
Here's another schema, very similar to the above:
create_table "posts", :force => true do |t|
t.string "name", :null => false
end
create_table "post_connections", :force => true do |t|
t.integer "post_a_id", :null => false
t.integer "post_b_id", :null => false
t.string "category"
end
Notice how, in this situation, post_connections does have an id column. (There's no :id => false parameter.) This is required, because there'll be a regular ActiveRecord model for accessing the table.
I'll start with the PostConnection model, because it's dead simple:
class PostConnection < ActiveRecord::Base
belongs_to :post_a, :class_name => :Post
belongs_to :post_b, :class_name => :Post
end
The only thing going on here is :class_name, which is necessary, because Rails cannot infer from post_a or post_b that we're dealing with a Post here. We have to tell it explicitly.
Now the Post model:
class Post < ActiveRecord::Base
has_many :post_connections, :foreign_key => :post_a_id
has_many :posts, :through => :post_connections, :source => :post_b
end
With the first has_many association, we tell the model to join post_connections on posts.id = post_connections.post_a_id.
With the second association, we are telling Rails that we can reach the other posts, the ones connected to this one, through our first association post_connections, followed by the post_b association of PostConnection.
There's just one more thing missing, and that is that we need to tell Rails that a PostConnection is dependent on the posts it belongs to. If one or both of post_a_id and post_b_id were NULL, then that connection wouldn't tell us much, would it? Here's how we do that in our Post model:
class Post < ActiveRecord::Base
has_many(:post_connections, :foreign_key => :post_a_id, :dependent => :destroy)
has_many(:reverse_post_connections, :class_name => :PostConnection,
:foreign_key => :post_b_id, :dependent => :destroy)
has_many :posts, :through => :post_connections, :source => :post_b
end
Besides the slight change in syntax, two real things are different here:
The has_many :post_connections has an extra :dependent parameter. With the value :destroy, we tell Rails that, once this post disappears, it can go ahead and destroy these objects. An alternative value you can use here is :delete_all, which is faster, but will not call any destroy hooks if you are using those.
We've added a has_many association for the reverse connections as well, the ones that have linked us through post_b_id. This way, Rails can neatly destroy those as well. Note that we have to specify :class_name here, because the model's class name can no longer be inferred from :reverse_post_connections.
With this in place, I bring you another irb session through script/console:
>> a = Post.create :name => 'Eels are delicious!'
=> #<Post id: 16, name: "Eels are delicious!">
>> b = Post.create :name => 'You insensitive cloth!'
=> #<Post id: 17, name: "You insensitive cloth!">
>> b.posts = [a]
=> [#<Post id: 16, name: "Eels are delicious!">]
>> b.post_connections
=> [#<PostConnection id: 3, post_a_id: 17, post_b_id: 16, category: nil>]
>> connection = b.post_connections[0]
=> #<PostConnection id: 3, post_a_id: 17, post_b_id: 16, category: nil>
>> connection.category = "scolding"
=> "scolding"
>> connection.save!
=> true
Instead of creating the association and then setting the category separately, you can also just create a PostConnection and be done with it:
>> b.posts = []
=> []
>> PostConnection.create(
?> :post_a => b, :post_b => a,
?> :category => "scolding"
>> )
=> #<PostConnection id: 5, post_a_id: 17, post_b_id: 16, category: "scolding">
>> b.posts(true) # 'true' means force a reload
=> [#<Post id: 16, name: "Eels are delicious!">]
And we can also manipulate the post_connections and reverse_post_connections associations; it will neatly reflect in the posts association:
>> a.reverse_post_connections
=> #<PostConnection id: 5, post_a_id: 17, post_b_id: 16, category: "scolding">
>> a.reverse_post_connections = []
=> []
>> b.posts(true) # 'true' means force a reload
=> []
Bi-directional looped associations
In normal has_and_belongs_to_many associations, the association is defined in both models involved. And the association is bi-directional.
But there is just one Post model in this case. And the association is only specified once. That's exactly why in this specific case, associations are uni-directional.
The same is true for the alternative method with has_many and a model for the join table.
This is best seen when simply accessing the associations from irb, and looking at the SQL that Rails generates in the log file. You'll find something like the following:
SELECT * FROM "posts"
INNER JOIN "post_connections" ON "posts".id = "post_connections".post_b_id
WHERE ("post_connections".post_a_id = 1 )
To make the association bi-directional, we'd have to find a way to make Rails OR the above conditions with post_a_id and post_b_id reversed, so it will look in both directions.
Unfortunately, the only way to do this that I know of is rather hacky. You'll have to manually specify your SQL using options to has_and_belongs_to_many such as :finder_sql, :delete_sql, etc. It's not pretty. (I'm open to suggestions here too. Anyone?)
To answer the question posed by Shteef:
Bi-directional looped associations
The follower-followee relationship among Users is a good example of a Bi-directional looped association. A User can have many:
followers in its capacity as followee
followees in its capacity as follower.
Here's how the code for user.rb might look:
class User < ActiveRecord::Base
# follower_follows "names" the Follow join table for accessing through the follower association
has_many :follower_follows, foreign_key: :followee_id, class_name: "Follow"
# source: :follower matches with the belong_to :follower identification in the Follow model
has_many :followers, through: :follower_follows, source: :follower
# followee_follows "names" the Follow join table for accessing through the followee association
has_many :followee_follows, foreign_key: :follower_id, class_name: "Follow"
# source: :followee matches with the belong_to :followee identification in the Follow model
has_many :followees, through: :followee_follows, source: :followee
end
Here's how the code for follow.rb:
class Follow < ActiveRecord::Base
belongs_to :follower, foreign_key: "follower_id", class_name: "User"
belongs_to :followee, foreign_key: "followee_id", class_name: "User"
end
The most important things to note are probably the terms :follower_follows and :followee_follows in user.rb. To use a run of the mill (non-looped) association as an example, a Team may have many :players through :contracts. This is no different for a Player, who may have many :teams through :contracts as well (over the course of such Player's career). But in this case, where only one named model exists (i.e. a User), naming the through: relationship identically (e.g. through: :follow, or, like was done above in the posts example, through: :post_connections) would result in a naming collision for different use cases of (or access points into) the join table. :follower_follows and :followee_follows were created to avoid such a naming collision. Now, a User can have many :followers through :follower_follows and many :followees through :followee_follows.
To determine a User’s :followees (upon an #user.followees call to the database), Rails may now look at each instance of class_name: “Follow” where such User is the the follower (i.e. foreign_key: :follower_id) through: such User’s :followee_follows. To determine a User’s :followers (upon an #user.followers call to the database), Rails may now look at each instance of class_name: “Follow” where such User is the the followee (i.e. foreign_key: :followee_id) through: such User’s :follower_follows.
If anyone came here to try to find out how to create friend relationships in Rails, then I would refer them to what I finally decided to use, which is to copy what 'Community Engine' did.
You can refer to:
https://github.com/bborn/communityengine/blob/master/app/models/friendship.rb
and
https://github.com/bborn/communityengine/blob/master/app/models/user.rb
for more information.
TL;DR
# user.rb
has_many :friendships, :foreign_key => "user_id", :dependent => :destroy
has_many :occurances_as_friend, :class_name => "Friendship", :foreign_key => "friend_id", :dependent => :destroy
..
# friendship.rb
belongs_to :user
belongs_to :friend, :class_name => "User", :foreign_key => "friend_id"
Inspired by #Stéphan Kochen,
this could work for bi-directional associations
class Post < ActiveRecord::Base
has_and_belongs_to_many(:posts,
join_table: 'post_connections',
foreign_key: 'post_a_id',
association_foreign_key: 'post_b_id')
has_and_belongs_to_many(:reversed_posts,
class_namy: Post,
join_table: 'post_connections',
foreign_key: 'post_b_id',
association_foreign_key: 'post_a_id')
end
then post.posts && post.reversed_posts should both works, at least worked for me.
For bi-directional belongs_to_and_has_many, refer to the great answer already posted, and then create another association with a different name, the foreign keys reversed and ensure that you have class_name set to point back to the correct model. Cheers.
If anyone had issues getting the excellent answer to work, such as:
(Object doesn't support #inspect)
=>
or
NoMethodError: undefined method `split' for :Mission:Symbol
Then the solution is to replace :PostConnection with "PostConnection", substituting your classname of course.