Question

First the data model:

class Forum < ActiveRecord::Base
  has_many :topics, :dependent => :destroy, :order => 'created_at desc'
end

class User < ActiveRecord::Base
  has_many :topics, :dependent => :destroy
  has_many :comments, :dependent => :destroy
  has_many :replies, :dependent => :destroy
end

class Topic < ActiveRecord::Base
  belongs_to :forum
  belongs_to :user
  has_many :comments, :dependent => :destroy
end

class Comment < ActiveRecord::Base
  belongs_to :user
  belongs_to :topic
  has_many :replies, :dependent => :destroy
end

class Reply < ActiveRecord::Base
  belongs_to :user
  belongs_to :comment
end

So Users can post Topics to Forums. They can also post Comments to the
Topics in a Forum. And they can post Replies to the Comments.

I want to be able to get a list of Forums they’ve participated in by
posting either Topics or Comments or Replies.

Answer 1

What you’re looking for is a named scope in the Forum model.

The join can be greatly simplified by adding has_many :through relationships for your Comment and Reply models in the Forum model. But I can never remember how nested joins work out, so I’ve posted a solution that will work with what you’ve got.

class Forum < ActiveRecord::Base
  has_many :topics, :dependent => :destroy, :order => 'created_at desc'
  named_scope :user_posted, lambda {|user|
    { :joins => "JOIN topics t ON t.forum_id = forums.id " +
        "JOIN comments c ON c.topic_id = t.id " +
        "JOIN replies r ON r.comment_id = c.id", 
      :conditions => ["t.user_id = ? OR c.user_id = ? OR r.user_id = ?", 
        user, user, user], 
      :group => "forums.id"
    }
  }
end

Now…

Forum.user_posted(@user)

Will return an array of forums where a user has posted a topic, reply or comment.

For a list of forums a particular user has posted in:

class User < ActiveRecord::Base
  has_many :topics, :dependent => :destroy
  has_many :comments, :dependent => :destroy
  has_many :replies, :dependent => :destroy    
  named_scope :posted_in_forum, lambda {|forum|
    { :joins => "JOIN replies r ON r.user_id = users.id "+
        "JOIN comments c ON c.user_id = users.id OR c.id = r.comment_id " +
        "JOIN topics t ON t.user_id = users.id OR t.id = c.topic_id " +
        "JOIN forums f ON t.forum_id = forums.id ",
      :conditions => ["f.id = ?", forum], 
      :group => "users.id"
    }
  }
end

Assuming I got that right, this statement:

User.posted_in_forum(@forum)

will return a list of users that have posted in forum either by topic, comment or reply.

P.S. it’s probably not a good idea to allow destruction of users in this model. The way you’ve laid things out, if a user is destroyed, any topics, replies or comments they posted will also be removed. Instead your users should be deactivated.