Self-referential find in controller count relations

问题

I'm having real trouble pulling out a set of records that are self-referentially related to a user in order to show these on a user's 'show' page.

Here's the idea:

Users (current_user) rate the compatibility between two other users (user_a and user_b). They can rate compatibility either positively or negatively: rating two users "compatible" creates a positive_connection between user_a and user_b, and rating them "incompatible" creates a negative_connection. So there are models for positive_connection, negative_connection and user.

Now I need to display only users that are overall_positively_connected_to(@user) (i.e. where positive_connections_to(@user).count > negative_connections_to(@user).count).

This is where I've got to, but I can't get any further:

User model:

  def overall_positive_connected_to(user)
      positive_connections_to(user).count > negative_connections_to(user).count
  end


  def positive_connections_to(user)
      positive_connections.where("user_b_id = ?", user)
  end     

  def negative_connections_to(user) 
      negative_connections.where("user_b_id = ?", user)
  end

Controller

@user.user_bs.each do |user_b|
  if user_b.overall_pos_connected_to(@user)
    @compatibles = user_b
  end
end

The code in the controller is clearly wrong, but how should I go about doing this? I'm completely new to rails (and sql), so may have done something naive.

Any help would be great.

回答1:

So am I right in saying you have 3 models

• User (id, name)
• PositiveConnection (user_a_id, user_b_id)
• NegativeConnection (user_a_id, user_b_id)

Or something of that sort.

I think you just want 2 models and for convenience I'm going to rename the relations as "from_user" and "to_user"

• User (id, name)
• Connection (value:integer, from_user_id, to_user_id)

Where value is -1 for a negative and +1 for a positive.

Now we can have do something like (note: you need to sort out the exact syntax, like :foreign_key, and :source, and stuff)

class User

  has_many :connections, :foreign_key => "from_user_id"
  has_many :connected_users, :through => :connections, :source => :to_user

  def positive_connections
    connections.where(:value => 1)
  end

  def negative_connections
    ...
  end

end

But we also now have a framework to create a complex sql query (again you need to fill in the blanks... but something like)

class User

  def positive_connected_users
    connected_users.joins(:connections).group("from_user_id").having("SUM(connections.value) > 0")
  end

end

this isn't quite going to work but is kind of pseudo code for a real solution

(it might be better to think in pure sql terms)

SELECT users.* FROM users
INNER JOIN connections ON to_user_id = users.id
WHERE  from_user_id = #{user.id}
HAVING SUM(connections.value) > 0

来源：https://stackoverflow.com/questions/6314135/self-referential-find-in-controller-count-relations