Rails 3 find all associated records has_many :through - ruby-on-rails

Question

I would like to list all posts that are connected with some specific category and classroom.
I have:
class Post < ActiveRecord::Base
has_many :category_posts
has_many :categories, :through => :category_posts
has_many :classroom_posts
has_many :classrooms, :through => :classroom_posts
end
class Category < ActiveRecord::Base
has_many :category_posts
has_many :posts, :through => :category_posts
end
class CategoryPost < ActiveRecord::Base
belongs_to :category
belongs_to :post
end
class Classroom < ActiveRecord::Base
has_many :classroom_posts
has_many :posts, :through => :classroom_posts
end
class ClassroomPost < ActiveRecord::Base
belongs_to :classroom
belongs_to :post
end
And I wanna do something like this
Post.where(["category.id = ? AND classroom.id = ?", params[:category_id], params[:classroom_id]])
It indeed is very simple task, but I don't know what I should be looking for (keywords).
It's the same problem like this, but in rails.
EDIT:
I added more details to the question.
This works, but only if I have both params specified. Witch is not always the case - I dont know what params would be specified.
Post.joins(:categories, :classrooms).where(["categories.id = ? AND classrooms.id = ?", params[:classroom_id], params[:category_id]])

Answer 1

Category.find(params[:category_id]).posts
Also take a look at the guides:
Guides for Rails 2.35
Guides for Rails 3.0

Answer 2

Here is what I would do in Rails 3:
In post.rb:
def self.in_category(category_id)
if category_id.present?
join(:category_posts).where(category_posts: {category_id: category_id})
else
self
end
end
def self.in_classroom(classroom_id)
if classroom_id.present?
join(:classroom_posts).where(classroom_posts: {classroom_id: category_id})
else
self
end
end
I do not join Classroom or Category since it makes more work for DBMS and this is not required.
Now, you can do:
Post.in_category(params[:category_id]).in_classroom(params[:classroom_id])
I haven't tested it though. So do not hesitated to ask if needed.

Answer 3

I think that should work:
Post.joins(:category_posts, :classroom_posts)
.where(
["category_posts.category_id = ?
AND classroom_posts.classroom_id = ?", params[:category_id], params[:classroom_id]])
This will traslate to a SQL like :
SELECT
p.*
FROM
posts AS p
INNER JOIN
category_posts AS cap ON cap.id = p.category_posts_id
INNER JOIN
classroom_posts AS clp ON clpid = p.classroom_posts_id
WHERE
cap.category_id = '1' AND clp.classroom_id = '1'
;
As to whether to use :include or joins on Post look at this answer on stackoverflow.

Answer 4

Sounds like you need an if statment.
if params[:category_id] && params[:classroom_id]
Post.joins(:categories, :classrooms).where("classrooms.id" => params[:classroom_id], "categories.id" => params[:category_id]])
elsif params[:category_id]
Category.find(params[:category_id]).posts
else
Classroom.find(params[:classroom_id]).posts
end