I'm trying to get a grasp of how to work with associations in Rails, specifically, when and when not to write explicit SQL code.
In my application, I have four models, which are defined as follows:
class User < ActiveRecord::Base
has_many :comments
has_many :geographies
has_many :communities, through: :geographies
class Comment < ActiveRecord::Base
belongs_to :user
class Community < ActiveRecord::Base
has_many :geographies
has_many :users
class Geography < ActiveRecord::Base
belongs_to :user
belongs_to :community
Users can post comments, and are associated to one or more communities through the geography table (the geography table stores user_id and community_id).
I have an index action listing all comments, and I would like to filter by community. Given a comment object, I can get the user object via comment.user, but I can't chain beyond that (i.e., something like comment.user.geography(0).community doesn't work).
It seems this object chaining is a key feature of rails, but does it work with has_many :through associations? Given my example, is it possible to get the community object from the comment object by using object chaining, or would I need to write the SQL to get anything other than the user when given the comment object?
Since User is associated with multiple communities, you will need to tell ActiveRecord (or raw SQL) which community you want:
comment.user.communities #=> should give you all the communities
If you don't particularly care for getting all communities and just want to get any community
comment.user.communities.first #=> should give you the first community
But, generally you will be interested in one particular community, based on a condition.
comment.user.communities.where(name: 'Europe') #=> should give you the European community.
I don't think you need the geographies table.
Try
class Community < ActiveRecord::Base
has_many :users
end
class User < ActiveRecord::Base
belongs_to :community
has_many :comments
end
class Comment < ActiveRecord::Base
belongs_to :user
end
Then you can access a comment's user's community like
#comment.user.community
Related
What is the difference between using just belongs_to on one model versus having has_many on one and belongs_to on another?
As an example:
class Author < ActiveRecord::Base
end
class Book < ActiveRecord::Base
belongs_to :author
end
versus
class Author < ActiveRecord::Base
has_many :books
end
class Book < ActiveRecord::Base
belongs_to :author
end
Thank you.
guessing each of the methods would facilitate adding a different set of additional methods to the associated class
for ex, if had to guess, with belongs_to, you would, in part, get ability to call association on an instance of Book:
#book.author
with has_many, if I had to guess, you would, in part, be able to call association on instance of Author:
#author.books
also, http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-belongs_to
and
http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_many
in case those may be of interest
Let's imagine that I have a CPA tracking system.
I would have following models: an Offer, it has some Landings, each of them has multiple Links, each of the links has a bunch of Visits.
So, I what I want is DRY code, therefore offer_id column within visits table is unacceptable. The workaround here is delegated methods like this:
class Offer < ActiveRecord::Base
has_many :landings
has_many :links, through: :landings
has_many :visits, through: :landings
end
class Landing < ActiveRecord::Base
belongs_to :offer
has_many :links
has_many :visits, through: :links
end
class Link < ActiveRecord::Base
belongs_to :landing
has_many :visits
delegate :offer, to: :landing
end
class Visit < ActiveRecord::Base
belongs_to :link
delegate :landing, to: :link
delegate :offer, to: :link
end
It works nice with a single visit, e.g. visit.offer.id. But what if I need different visits associated with one offer?
The issue is that I'm unable to construct a valid query using ActiveRecord API. It might look like Visits.where(offer: Offer.first), but it doesn't work this way, saying ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: visits.offer: SELECT "visits".* FROM "visits" WHERE "visits"."offer" = 1, which is predictable.
Question: How should I organize my code to make statements like Visits.where(offer: Offer.first) work efficiently without duplicating offer_id column within visits table?
You code was organized nicely, don't need to refactor I think. You can achieve that by defining a scope in Visit like this:
class Visit < ActiveRecord::Base
scope :from_offer, -> (offer) {
joins(link: :landing).where(ladings: {offer_id: offer.id})
}
scope :from_landing, -> (landing) {
joins(:link).where(links: {landing_id: landing.id})
}
end
So the query will be:
Visit.from_offer(Offer.first)
I'm adding quiz functionality to the twitter app from the Hartl tutorial and have these Models:
User is nearly the same as the tutorial:
class User < ActiveRecord::Base
has_many :followed_users, through: :relationships, source: :followed
has_many :takens, dependent: :destroy
has_many :questions, through: :takens
end
Taken is a table of Question ids to User ids:
class Taken < ActiveRecord::Base
belongs_to :user
belongs_to :question
end
nothing interesting in Question:
class Question < ActiveRecord::Base
attr_accessible :category, :correct, :option1, :option2, :option3, :qn
end
I want to be able to show followed_users and followers in order of the number of tests they have taken. In the console this can be had through:
User.find_by_id(1).question_ids.count
Then I can do something like:
User.find_by_id(1).followers.first.question_ids.count
in the console to get the count for a single follower.
I feel like I'm almost there.
How do I sort the followers and followed_users through their 'takens' count? (I was also looking at cache_count, which at first seemed promising, but might not be what I need...)
Ruby on Rails does not provide an object oriented mechanism to perform this; you have to write the SQL yourself. In your case, I'd say that the following line SHOULD work:
User.find_by_sql("SELECT users.*, COUNT(questions.id)
AS c FROM users, questions WHERE questions.user_id = users.id
GROUP BY users.id ORDER BY c DESC")
I don't have the actual tables in front of me, so I can't be sure that this is actual valid SQL, but hopefully it should work.
EDIT: There were a few syntax errors with my SQL but they've been fixed. Note that I'm assuming that your tables are called users and questions. They may differ for you.
I've reread the rails guide to associations but still am hitting my head up against the wall. I've seen some advanced questions on Stack Overflow and am not able to determine if they shed light on my problem. Perhaps I've haven't gotten my mind wrapped around this. Please show me what I'm not seeing.
I have a School.rb which has_many Events, has_many Venues. Each of the Events and Venues belongs_to School. I'm trying to link up the Venue to an Event. They are tied to the school because they have a matching school_id. The name of the school is easily applied in Event#show and Venue#show as expected. The trick is how do I craft the Event controller to use the school_id to pull the Venue's addy in the Event#show page?
My attempts keep missing so I got to thinking maybe I have to make the Event belongs_to Venue and Venue has_many Events. Is that the right thing to do?
I attempt <%= #event.venue.address %> but that fails with 'undefined method `address' for nil:NilClass'
Maybe I'm overthinking it but as I mention above, I don't know enough to ask the right question. If I was to put my query in English terms it would be "Grab the instance of Venue whose school_id matches the school_id of the current/active Event." Does that make sense? I've attempted to find something close to that in the rails guides and attempted this:
#venues = Venue.where(:school_id => #school_id)
undefined method `address' for []:ActiveRecord::Relation. The venue address is in the venue model.
Here's my school.rb:
class School < ActiveRecord::Base
has_many :events, :dependent => :destroy
has_many :venues, :dependent => :destroy
has_and_belongs_to_many :users
belongs_to :event belongs_to :venue
def self.fetch_for_name(_name)
school = self.new(:name => _name)
end
end
Here's my event.rb:
class Event < ActiveRecord::Base
resourcify
belongs_to :school
belongs_to :venue
end
Here's my Venue.rb:
class Venue < ActiveRecord::Base
resourcify
belongs_to :school
has_many :events
end
Please help me get over this baby step, sam
You need to look at this part of the guide and ensure you have has_many through relationship between venue and events here as explained in the link...
To do the exact thing you are asking, you'd create a method in Venue to query for a given event.
class Venue < ActiveRecord::Base
resourcify
belongs_to :school
has_many :events
def self.venue_for_event(event)
where("school_id = ?", event.school_id)
end
end
However, there are some questions to be asked about your models. Why does a school have many events and venues yet also belong to one event and venue? What is the problem you are trying to solve with these models? If school is the glue that holds venues and events together, than consider making it a join model in a has_many, :through relationshuip
Going back to an example I had previously asked about, I'll try and make this question as simple as possible.
Supposed I have User and Document models.
A User has_many Documents and a Document belongs_to a User. This relationship works fine today.
I want to introduce an Edits model that belongs_to a Document and belongs_to a User. The User can be any user, not necessary the one who created the document. With this new model, the Document now has_many :edits and the User has_many :edits.
So far it would look like:
# user.rb
class User < ActiveRecord::Base
has_many :edits
has_many :documents
end
# document.rb
class Document < ActiveRecord::Base
belongs_to :user
has_many :edits
end
# edit.rb
class Edit < ActiveRecord::Base
belongs_to :user
belongs_to :document
end
When I create a Document through a User, the association between the User and Document are fine in both directions (user.documents and document.user)
Now when I want to create an Edit, the edit should be against a Document but should also be associated with the user who generated the edit (edit.user).
When I'm building this in my RSpec tests I'm struggling to get the associations correct using the "standard" association methods. If I do #user.edits.build({...}) it will associate the user in the returned Edit object, but not the Document. Likewise when I do #document.edits.build({...}) it will associate the Document but not the User.
I supposed I could expose the user_id and post_id in the attr_accessible declaration but won't this but I fear this is not a best way of doing this. I have no real reason to fear other than the attributes are now accessible through mass assignment (from what I understand).
Am I going about this the wrong way or is there a better way to create and test all the associations?
class User < ActiveRecord::Base
has_many :edits, :through => :documents
has_many :documents
end