Question

I want to obtain Bar records associated to a single foo through bars in the following models,
Foo <-> Bar <-> Baz.

class Foo < ActiveRecord::Base
  has_many :foo_bar_assocs
  has_many :bars, through: :foo_bar_assocs
end

class Bar < ActiveRecord::Base
  has_many :bar_baz_assocs
  has_many :bazs, through: :bar_baz_assocs
  attr_readonly :name
end

class Baz < ActiveRecord::Base
end

class FooBarAssoc < ActiveRecord::Base
  belongs_to :foo
  belongs_to :bar
end

class BarBazAssoc < ActiveRecord::Base
  belongs_to :bar
  belongs_to :baz
end

Naive implementation:

foo = Foo.find(id)
baz_of_foo = foo.bars.where(name: params[:name]).map{|b| b.bazs}.flatten.uniq

This code generates bad query, it executes many times.
What I want is something like the following:

baz_of_foo = foo.something_good_query(bar_name: params[:name])

Also, I want to get objects like ActiveRecord::Relation so that the resulting SQL queries are optimized.

baz_of_foo.each{ ... } # SELECT DISTINCT baz.* FROM ...
baz_of_foo.count       # SELECT COUNT(DISTINCT baz.*) FROM ...
baz_of_foo.exists?     # SELECT baz.* FROM ... TAKE 1

Answer 1

You should define the reverse relationships too, once you’ve done that you should be able to do something like this:

Baz.joins(:bar => :foo).where(:bar => {:name => params[:name], :foo => {:id => foo_id}})

For complex queries i would also recommend the squeel gem, it allows for more compact queries and also supports outer joins.