Question

I’m converting an app from php to rails, and I’m still learning my way around rails and AR.

Simply: I want to list the groups that the current user is not already a member of.

Failed Approach:

1. Cause.select('causes.*').joins(:users).group('causes.id').where("cause_user_memberships.user_id
NOT IN (?)", current_user.id)

2. Cause.select('causes.*').joins(:users).group('causes.id').where("cause_user_memberships.user_id NOT IN (SELECT cause_user_memberships.cause_id FROM cause_user_memberships WHERE cause_user_memberships.user_id =(?))", current_user.id)

3. Many more…

Thanks for your help!

Some info about models

User.rb (snippet)

has_many :cause_user_memberships
has_many :causes, :through => :cause_user_memberships

Cause.rb

attr_accessible :title, :location, :description,...
has_many :cause_user_memberships
has_many :users, :through => :cause_user_memberships

Cause_User_Membership.rb (<–probably not my best model name)

    # == Schema Information
#
# Table name: cause_user_memberships
#
#  id         :integer          not null, primary key
#  user_id    :integer          not null
#  cause_id   :integer          not null
#  created_at :datetime         not null
#  updated_at :datetime         not null
#

class CauseUserMembership < ActiveRecord::Base
  attr_accessible :cause_id, :user_id

  belongs_to :user 
  belongs_to :cause, :counter_cache => :users_count 
      accepts_nested_attributes_for :cause
      validates_uniqueness_of :user_id, :scope =>[:cause_id]

end

Update: Follow up

Derp, you’re right it worked! Thanks!

Small follow up, the query times seem pretty long. Does this indicate a problem? I have less than 20 records in each table. (Below are 2 query results, one that includes the geocoder gem I intend to use and another that doesn’t. Sorry if it’s a little messy.)

In Rails Console:

Cause Load (1003.0ms)  SELECT "causes".* FROM "causes" LEFT JOIN cause_user_memberships ON cause_user_memberships.cause_id = causes.id AND cause_user_memberships.user_id = 1 WHERE (cause_user_memberships.id IS NULL)

EXPLAIN (34.3ms)  EXPLAIN SELECT "causes".* FROM "causes" LEFT JOIN cause_user_memberships ON cause_user_memberships.cause_id = causes.id AND cause_user_memberships.user_id = 1 WHERE (cause_user_memberships.id IS NULL)

QUERY PLAN

 Hash Right Join  (cost=10.45..37.99 rows=1 width=3168)
   Hash Cond: (cause_user_memberships.cause_id = causes.id)
   Filter: (cause_user_memberships.id IS NULL)
   ->  Seq Scan on cause_user_memberships  (cost=0.00..27.50 rows=7 width=8)
         Filter: (user_id = 1)
   ->  Hash  (cost=10.20..10.20 rows=20 width=3168)
         ->  Seq Scan on causes  (cost=0.00..10.20 rows=20 width=3168)
(7 rows)

With Geocoder on local host:

User Load (18.0ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 1 LIMIT 1
  Cause Load (49.6ms)  SELECT causes.*, 3958.755864232 * 2 * ASIN(SQRT(POWER(SIN((40.714269 - causes.lat) * PI() / 180 / 2), 2) + COS(40.714269 * PI() / 180) * COS(causes.lat * PI() / 180) * POWER(SIN((-74.005972 - causes.lng) * PI() / 180 / 2), 2))) AS distance, CAST(DEGREES(ATAN2( RADIANS(causes.lng - -74.005972), RADIANS(causes.lat - 40.714269))) + 360 AS decimal) % 360 AS bearing FROM "causes" LEFT JOIN cause_user_memberships 
 ON cause_user_memberships.cause_id = causes.id 
 AND cause_user_memberships.user_id = 1 WHERE (causes.lat BETWEEN 36.37231550667456 AND 45.05622249332544 AND causes.lng BETWEEN -79.73435509229111 AND -68.27758890770889 AND 3958.755864232 * 2 * ASIN(SQRT(POWER(SIN((40.714269 - causes.lat) * PI() / 180 / 2), 2) + COS(40.714269 * PI() / 180) * COS(causes.lat * PI() / 180) * POWER(SIN((-74.005972 - causes.lng) * PI() / 180 / 2), 2))) <= 300) AND (cause_user_memberships.id IS NULL) ORDER BY distance ASC

Completed 200 OK in 1068ms (Views: 49.1ms | ActiveRecord: 791.4ms)

Answer 1

In rails the joins(:symbol) statement turns into an inner join which is not suited for finding relations that don’t exist. You can write the join by hand in order to make it a left join:

Cause.joins("""
  LEFT JOIN cause_user_memberships 
  ON cause_user_memberships.cause_id = causes.id 
  AND cause_user_memberships.user_id = #{current_user.id}
""").where("cause_user_memberships.id IS NULL") 

UPDATED

The inner join will prevent the creation of a set of Causes that don’t have a mapping to the given user. For example:

Causes
id | name
============
1  | Cause 1
2  | Cause 2

Users
id | name
============
1  | User 1

Cause User Memberships
id | user_id | cause_id
=======================
1  | 1       | 1

SELECT *
FROM causes
INNER JOIN cause_user_memberships
ON cause_user_memberships.cause_id = causes.id
AND cause_user_memberships.user_id = 1

Will return

causes.id | causes.name | cause_user_memberships.id | cause_user_memberships.user_id | cause_user_memberships.cause_id
===========================================================================================================================
1         | Cause 1     | 1                         | 1                              | 1

Which you can’t perform any more logic on in order to tease out the causes the user doesn’t belong to.

SELECT *
FROM causes
LEFT JOIN cause_user_memberships
ON cause_user_memberships.cause_id = cause.id
AND cause_user_memberships.user_id = 1

Will return

causes.id | causes.name | cause_user_memberships.id | cause_user_memberships.user_id | cause_user_memberships.cause_id
===========================================================================================================================
1         | Cause 1     | 1                         | 1                              | 1
2         | Cause 2     | null                      | null                           | null

In this case all the Causes will get a row whether they match a record in the cause_user_memberships table or not. Now you can apply additional conditions to tease out the causes the user doesn’t belong to (where cause_user_membership.id is null).