ActiveRecord Where Assoc

This gem makes it easy to do conditions based on the associations of your records in ActiveRecord (Rails). (Using SQL's EXISTS operator)

# Find my_post's comments that were not made by an admin
my_post.comments.where_assoc_not_exists(:author, is_admin: true).where(...)

# Find every posts that have comments by an admin
Post.where_assoc_exists([:comments, :author], &:admins).where(...)

# Find my_user's posts that have at least 5 non-spam comments (not_spam is a scope on comments)
my_user.posts.where_assoc_count(5, :>=, :comments) { |comments| comments.not_spam }.where(...)

These allow for powerful, chainable, clear and easy to reuse queries. (Great for scopes)

Here is an introduction to this gem.

You avoid many problems with the alternative options.

Here are many examples, including the generated SQL queries.

Advantages

These methods have many advantages over the alternative ways of achieving the similar results:

• Avoids the problems with the alternative ways
• Can be chained and nested with regular ActiveRecord methods (where, merge, scope, etc).
• Adds a single condition in the WHERE of the query instead of complex things like joins. So it's easy to have multiple conditions on the same association
• Handles has_one correctly: only testing the "first" record of the association that matches the default_scope and the scope on the association itself.
• Handles recursive associations (such as parent/children) seemlessly.
• Can be used to quickly generate a SQL query that you can edit/use manually.

Installation

Rails 4.1 to 7.0 are supported with Ruby 2.1 to 3.1. Tested against SQLite3, PostgreSQL and MySQL. The gem only depends on the activerecord gem.

Add this line to your application's Gemfile:

gem 'activerecord_where_assoc', '~> 1.0'

And then execute:

$ bundle install

Or install it yourself with:

$ gem install activerecord_where_assoc

Development state

This gem is feature complete and production ready.
Other than rare tweaks as new versions of Rails and Ruby are released, there shouldn't be much activity on this repository.

Documentation

The documentation is nicely structured

If you prefer to see it in the code, the main methods are in this file and the ones that return SQL parts are in this one

Here are some usage tips

Usage

You can view many examples.

Otherwise, here is a short explanation of the main methods provided by this gem:

where_assoc_exists(association_name, conditions, options, &block)
where_assoc_not_exists(association_name, conditions, options, &block)
where_assoc_count(left_operand, operator, association_name, conditions, options, &block)

• These methods add a condition (a #where) that checks if the association exists (or not)
• You can specify condition on the association, so you could check only for comments that are made by an admin.
• Each method returns a new relation, meaning you can chain #where, #order, limit, etc.
• common arguments:

  • association_name: the association we are doing the condition on.

  • conditions: (optional) the condition to apply on the association. It can be anything that #where can receive, so: Hash, String and Array (string with binds).

  • options: available options to alter some behaviors. (rarely necessary)

  • block: adds more complex conditions by receiving a relation on the association. Can use #where, #where_assoc_*, scopes, and other scoping methods.
    Must return a relation. The block either:

    • receives no argument, in which case self is set to the relation, so you can do { where(id: 123) }
    • receives arguments, in which case the block is called with the relation as first parameter.

    The block should return the new relation to use or nil to do as if there were no blocks.
    It's common to use where_assoc_*(..., &:scope_name) to use a single scope.

• #where_assoc_count is a generalization of #where_assoc_exists and #where_assoc_not_exists. It behaves the same way, but is more powerful, as it allows you to specify how many matches there should be.

  # These are equivalent:
  Post.where_assoc_exists(:comments, is_spam: true)
  Post.where_assoc_count(1, :<=, :comments, is_spam: true)
  
  Post.where_assoc_not_exists(:comments, is_spam: true)
  Post.where_assoc_count(0, :==, :comments, is_spam: true)
  
  # This has no equivalent (Posts with at least 5 spam comments)
  Post.where_assoc_count(5, :<=, :comments, is_spam: true)

• where_assoc_count's additional arguments
  The order of the parameters of #where_assoc_count may seem confusing, but you will get used to it. It helps to remember: the goal is to do: 5 < (SELECT COUNT(*) FROM ...), the number is first, then operator, then the association and its conditions.
  • left_operand:
    • a number
    • a string of SQL to embed in the query
    • a range (operator must be :== or :!=)
      will use SQL's BETWEEN or NOT BETWEEN
      supports infinite ranges and exclusive end
  • operator: one of :<, :<=, :==, :!=, :>=, :>

Intuition

Here is the basic intuition for the methods:

#where_assoc_exists filters the models, returning those where a record for the association matching a condition (by default any record in the association) exists.

#where_assoc_not_exists is the exact opposite of #where_assoc_exists. Filters the models, returning those where a record for the association matching a condition (by default any record in the association) do not exists

#where_assoc_count the more specific version of #where_assoc_exists. Filters the models, returning those where a record for the association matching a condition (by default any record in the association) do not exists

The condition that you may need on the record can be quite complicated. For this reason, you can pass a block to these methods.
The block will receive a relation on records of the association. Your job is then to call where and scopes to specify what you want to exist (or to not exist if using #where_assoc_not_exists).

So if you have User.where_assoc_exists(:comments) {|rel| rel.where("content ilike '%github.com%'") }, rel is a relation is on Comment, and you are specifying what you want to exist. So now we are looking for users that made a comment containing 'github.com'.

Usage tips

Nested associations

Sometimes, there isn't a single association that goes deep enough. In that situation, you can simply nest the scopes:

# Find users that have a post that has a comment that was made by an admin.
# Using &:admins to use the admins scope (or any other class method of comments)
User.where_assoc_exists(:posts) { |posts|
    posts.where_assoc_exists(:comments) { |comments| 
        comments.where_assoc_exists(:author, &:admins)
    }
}

If you don't need special conditions on any of the intermediary associations, then you can an array as shortcut for multiple steps:

# Same as above
User.where_assoc_exists([:posts, :comments, :author], &:admins)

This shortcut can be used for every where_assoc_* methods. The conditions and the block will only be applied to the last association of the chain.

Beware of spreading conditions on multiple calls

The following have different meanings:

my_user.posts.where_assoc_exists(:comments_authors, is_admin: true, is_honest: true)

my_user.posts.where_assoc_exists(:comments_authors, is_admin: true)
             .where_assoc_exists(:comments_authors, is_honest: true)

The first is the posts of my_user that have a comment made by an honest admin. It requires a single comment to match every conditions.

The second is the posts of my_user that have a comment made by an admin and a comment made by someone honest. It could be the same comment (like the first query) but it could also be 2 different comments.

Inter-table conditions

It's possible, with string conditions, to refer to all the tables that are used before the association, including the source model.

# Find posts where the author also commented on the post.
Post.where_assoc_exists(:comments, "posts.author_id = comments.author_id")

Note that some database systems limit how far up you can refer to tables in nested queries. Meaning it's possible that the following query may get refused because of those limits:

# it's hard to come up with a good example...
Post.where_assoc_exists([:comments, :author, :address], "addresses.country = posts.database_country")

Doing the same thing but with less associations between address and posts would not be an issue.

Getting SQL strings

Sometimes, you may need only the SQL of the condition instead of a whole relation, such as when writing your own complex SQL. There are methods available for this use case: assoc_exists_sql, assoc_not_exists_sql, compare_assoc_count_sql, only_assoc_count_sql.

You can read some more about them in their documentation

Here is a simple example of they use. Note that they should always be called on the class.

    # Users with a post or a comment
    User.where("#{User.assoc_exists_sql(:posts)} OR #{User.assoc_exists_sql(:comments)}")
    my_users.where("#{User.assoc_exists_sql(:posts)} OR #{User.assoc_exists_sql(:comments)}")
    # Note that this could be achieved in Rails 5 using the #or method and #where_assoc_exists

The opposite of multiple nested EXISTS...

... is a single NOT EXISTS with the nested ones still using EXISTS.

All the methods always chain nested associations using an EXISTS when they have to go through multiple hoops. Only the outer-most, or first, association will have a NOT EXISTS when using #where_assoc_not_exists or a COUNT when using #where_assoc_count. This is the logical way of doing it.

Using #from in scope

If you want to use a scope / condition which uses #from, then you need to use the :never_alias_limit option to avoid #where_assoc_* being overwritten by your scope and getting a weird exception / wrong result.

Known issues/limitations

MySQL doesn't support sub-limit

On MySQL databases, it is not possible to use has_one associations and associations with a scope that apply either a limit or an offset.

I do not know of a way to do a SQL query that can deal with all the specifics of has_one for MySQL. If you have one, then please suggest it in an issue/pull request.

In order to work around this, you must use the ignore_limit option. The behavior is less correct, but better than being unable to use the gem.

has_* :through vs limit/offset

For has_many and has_one with the :through option, #limit and #offset are ignored. Note that #limit and #offset of the :source and of the :through side are applied correctly.

This is the opposite of what ActiveRecord does when you fetch the result of such an association. ActiveRecord will ignore the limits of the part :source and of the :through and only use the one of the has_* :through.

It is pretty complicated to support #limit and #offset of the has_* :through and would require quite a bit of refactoring. PR welcome

Note that the support of #limit and #offset for the :source and :through parts is a feature. I consider ActiveRecord wrong for not handling them correctly.

Another recommended gem

If you feel a need for this gem's feature, you may also be interested in another gem I made: activerecord_follow_assoc.

It allows you to follow an association of your choice while building a query (a scope). You start querying posts, and then you change to querying the authors of those posts. For simple cases, it's possible that both where_assoc and follow_assoc can build the query your need, but each can handle different situations. Here is an example:

# Find every posts that have comments by an admin
Post.where_assoc_exists([:comments, :author], &:admins)

This could be done with follow_assoc: User.admins.follow_assoc(:comments, :post). But if you wanted conditions on a second association, then follow_assoc wouldn't work. On the other hand, if you received a scope on users and wanted their posts, then follow_assoc would be a nicer tool for the job. It all depends on the context where you need to do the query and what starting point you have.

Development

After checking out the repo, run bundle install to install dependencies.

Run rake test to run the tests for the latest version of rails. If you want SQL queries printed when you have failures, use SQL_WITH_FAILURES=1 rake test.

Run bin/console for an interactive prompt that will allow you to experiment in the same environment as the tests.

Run bin/fixcop to fix a lot of common styling mistake from your changes and then display the remaining rubocop rules you break. Make sure to do this before committing and submitting PRs. Use common sense, sometimes it's okay to break a rule, add a rubocop:disable comment in that situation.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/MaxLap/activerecord_where_assoc.

License

The gem is available as open source under the terms of the MIT License.

Acknowledgements

• René van den Berg for some of the code of activerecord-like used for help with setting up the tests