Rails way to harness the power of SQL EXISTS condition

This gem does exactly two things:

• Selects each model object for which there is a certain associated object
• Selects each model object for which there aren't any certain associated objects

It uses SQL EXISTS condition to do it fast, and extends ActiveRecord with where_exists and where_not_exists methods to make its usage simple and straightforward.

Add gem to Gemfile:

gem 'where_exists'

and run bundle install as usual.

And now you have where_exists and where_not_exists methods available for your ActiveRecord models and relations.

Syntax:

Model.where_exists(association, additional_finder_parameters)

Supported Rails versions: >= 3.2.22.

Given there is User model:

class User < ActiveRecord::Base
  has_many :connections
  has_many :groups, through: :connections
end

And Group:

class Group < ActiveRecord::Base
  has_many :connections
  has_many :users, through: :connections
end

And standard many-to-many Connection:

class Connection
  belongs_to :user
  belongs_to :group
end

What I want to do is to:

• Select users who don't belong to given set of Groups (groups with ids [4,5,6])
• Select users who belong to one set of Groups ([1,2,3]) and don't belong to another ([4,5,6])
• Select users who don't belong to a Group

Also, I don't want to:

• Fetch a lot of data from database to manipulate it with Ruby code. I know that will be inefficient in terms of CPU and memory (Ruby is much slower than any commonly used DB engine, and typically I want to rely on DB engine to do the heavy lifting)
• I tried queries like User.joins(:group).where(group_id: [1,2,3]).where.not(group_id: [4,5,6]) and they return wrong results (some users from the result set belong to groups 4,5,6 as well as 1,2,3)
• I don't want to do join merely for the sake of only checking for existence, because I know that that is a pretty complex (i.e. CPU/memory-intensive) operation for DB

_{^{If you wonder how to do that without the gem (i.e. essentially by writing SQL EXISTS statement manually) see that StackOverflow answer (disclosure: it's self-answered question of a contributor of this gem).}}

And now you are able to do all these things (and more) as simple as:

Select only users who don't belong to given set of Groups (groups with ids [4,5,6])

# It's really neat, isn't it?
User.where_exists(:groups, id: [4,5,6])

_{^{Notice that the second argument is where parameters for Group model}}

Select only users who belong to one set of Groups ([1,2,3]) and don't belong to another ([4,5,6])

# Chain-able like you expect them to be.
#
# Additional finder parameters is anything that
# could be fed to 'where' method.
#
# Let's use 'name' instead of 'id' here, for example.

User.where_exists(:groups, name: ['first','second','third']).
  where_not_exists(:groups, name: ['fourth','fifth','sixth'])

_{^{It is possible to add as much attributes to the criteria as it is necessary, just as with regular where(...)}}

Select only users who don't belong to a Group

# And that's just its basic capabilities
User.where_not_exists(:groups)

_{^{Adding parameters (the second argument) to where_not_exists method is feasible as well, if you have such requirements.}}

Q: Does it support both has_many and belongs_to association type?
A: Yes.

Q: Does it support polymorphic associations?
A: Yes, both ways.

Q: Does it support multi-level (recursive) :through associations?
A: You bet. (Now you can forget complex EXISTS or JOIN statetements in a pretty wide variety of similar cases.)

If you find that this gem lacks certain possibilities that you would have found useful, don't hesitate to create a feature request.

Also,

• Report bugs
• Submit pull request with new features or bug fixes
• Enhance or clarify the documentation that you are reading

This project uses MIT license. See MIT-LICENSE file for full text.