Let say I have a Group of Users and User has a enum, which described it's mood.
class Group < AR::Base
has_many :users
class User < AR::Base
belongs_to :group
enum mood: %i(good bad ugly)
How can I find all groups, where at least one User has the good mood?
Which index should I add for optimizing this query?
You can use joins method.
Group.joins(:users).where("users.mood = ?", User.moods[:good])
You can add this into your migration
add_index :users, :group_id
Related
In the below association, I want to collect all Users who don't have any projects->
class User < ActiveRecord::Base
has_many :projects, :foreign_key => :user_id
end
class Projects < ActiveRecord::Base
belongs_to :user, :foreign_key => "user_id"
end
From User model, how can I get all the users that do not have any projects? I tried using includes and join but didn't get the expected result
You could try:
User.where.not(id: Project.pluck(:user_id).uniq)
Breaking it down:
Project.pluck(:user_id).uniq
will give you an array of user_ids from your projects. Essentially, users with projects.
Then:
User.where.not(id: Project.pluck(:user_id).uniq)
returns users who have an id that is not in the array of users with projects.
I have a many to many :through relationship between a set of classes like so:
class Company
has_many :shares
has_many :users, :through => :shares, :uniq => true
end
class User
has_many :shares
has_many :companys, :through => :shares, uniq => true
end
class Share
belongs_to :company
belongs_to :user
end
I want to ensure a unique relationship so that a user can only have one share in any one company, which is what I have tried to achieve using the "uniq" argument.
At first I thought this was working, however it seems the behaviour os the "uniq" is to filter on the SELECT of the record, not pre-INSERT so I still get duplicate records in the database, which becomes an issue if I want to start dealing with the :shares association directly, as calling user.shares will return duplicate records if they exist.
Can anyone help with an approach which would force truely uniq relationships? so that if I try adding the second relationships between a user and a company it will reject it and only keep the original?
Have you tried adding this to your Share class?
validates_uniqueness_of :user, scope: :company
Also, in your User class I think it should be:
has_many :companies, through: :shares
I hope that helps.
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
I'm trying to do a basic model association in rails.
Basically I have a List table which stores item_id and user_id.
One user can create multiple "list-items."
Is this the correct way to do it?
Thanks.
class Item < ActiveRecord::Base
has_many :users, :through => :lists
end
class User < ActiveRecord::Base
has_many :items, :through => :lists
end
class List < ActiveRecord::Base
belongs_to :user
belongs_to :item
end
Depending on what you want to reach, your solution is the right one (or not). I see the following cases:
You want to create an n:m association between items and users. So each item could be referenced by many users, and each user references many items. If this is the right context, then your solution is the right one. See the Rails Guides: Associations for more information on that.
An alternative for that situation could be to use the has_and_belongs_to_many Association. The situation is the same, but it does not make sense to talk about lists, there will be no model object for it.
If each users may have many lists, and each list may have many items, your solution would be wrong. This would be no n:m association with list as the join table in between, but two 1:n relations.
The code for the third example would look like that:
class User < ActiveRecord::Base
has_many :items, :through => :lists
has_many :lists
end
class List < ActiveRecord::Base
has_many :items
belongs_to :user
end
class Item < ActiveRecord::Base
belongs_to :list
end
In the first solution, you should add the relations for users to lists and items to list:
class Item < ActiveRecord::Base
has_many :lists
has_many :users, :through => :lists
end
class User < ActiveRecord::Base
has_many :lists
has_many :items, :through => :lists
end
If the "list" entity truly is a pure association/join, that is, it has no inherent attributes of its own, then you can simplify a bit and use has_and_belongs_to_many. Then you don't need a "List" class.
class Item < ActiveRecord::Base
has_and_belongs_to_many :users
end
class User < ActiveRecord::Base
has_and_belongs_to_many :items
end
Rails will look for the references in a "items_users" table, so in your migration, you need to create it a la:
create_table :items_users, :id => false do |t|
t.references :users, :items
end
Many people will tell you to always use has_many :through, but others (like me) will disagree - use the right tool for job.
anyone know how I would write that query with AR?
select *, (m.user_id=1) as member from band b join memberships m on m.band_id = g.id;
Thanks in advance.
The assumption here is that you have something that looks like this:
class Band < ActiveRecord::Base
has_many :memberships
has_many :users, :through => :memberships
end
class User < ActiveRecord::Base
has_many :memberships
has_many :bands, :through => :memberships
end
class Membership < ActiveRecord::Base
belongs_to :user
belongs_to :band
end
In which case, you can perform this query easily.
user = User.find(1)
user.bands
Your question is quite vague, however, so if this isn't what you're looking for please considering expanding your question with some more details. You are also referencing an alias "g" in your question which is never defined.
I don't see why you would want to set boolean attributes via the SQL queries. You can do this with good ol' Ruby, which also lets you use has many through as jdl said.
class Book
def member?
user_id == 1
end
end