Question

I am working on a rails project for a while and I juste encountered (what I think is) a small problem.

To make it simple, I have 3 model like this:

 class House < ActiveRecord::Base
  has_many :house_style_relationships
 end

 class HouseStyleRelationship < ActiveRecord::Base
  attr_accessible :percentage

  belongs_to :style
  belongs_to :house
 end

class Style < ActiveRecord::Base
  has_many :house_style_relationships
end

So the HouseStyleRelationship model simply let us know the style percentage of an House.
For example an House can be 70% Modern and 30% Classic.

What I need to do is to get the average of each styles for all the houses.
To do that I used this working query under SQLite:

houses_count = Houses.count
HouseStyleRelationship.joins(:style).select("style.name, SUM(house_style_relationships.percentage)/#{houses_count} as percentage").group("styles.id")

But when I decided to push all this stuff on Heroku, I got a problem with PostgreSQL.
In fact, to create an HouseStyleRelationship object (I don’t even need), ActiveRecord ask for a HouseStyleRelationship.id which make the Group By crashing (with the query).
(PostgreSQL don’t want to group records with different ids)

Do you have a solution to prevent ActiveRecord to generate model instances as answer and so remove the HouseStyleRelationship.id from the query ?
(I can’t use SELECT DISTINCT since I need the SUM() calculation)

Thanks in advance !

Answer 1

You need to create your join table such that it has no primary key. The create_table statement in your migration should look something like this.

create_table :houses_style_relationships, :id => false do |t|