module ActiveRecord::Querying

# File activerecord/lib/active_record/querying.rb, line 78
def count_by_sql(sql)
  connection.select_value(sanitize_sql(sql), "#{name} Count").to_i
end

Returns the result of an SQL statement that should only include a COUNT(*) in the SELECT part. The use of this method should be restricted to complicated SQL queries that can't be executed using the ActiveRecord::Calculations class methods. Look into those before using this method, as it could lock you into a specific database engine or require a code change to switch database engines.

Product.count_by_sql "SELECT COUNT(*) FROM sales s, customers c WHERE s.customer_id = c.id"
# => 12

Parameters

• sql - An SQL statement which should return a count query from the database, see the example above.

# File activerecord/lib/active_record/querying.rb, line 45
def find_by_sql(sql, binds = [], preparable: nil, &block)
  result_set = connection.select_all(sanitize_sql(sql), "#{name} Load", binds, preparable: preparable)
  column_types = result_set.column_types.dup
  attribute_types.each_key { |k| column_types.delete k }
  message_bus = ActiveSupport::Notifications.instrumenter

  payload = {
    record_count: result_set.length,
    class_name: name
  }

  message_bus.instrument("instantiation.active_record", payload) do
    if result_set.includes_column?(inheritance_column)
      result_set.map { |record| instantiate(record, column_types, &block) }
    else
      # Instantiate a homogeneous set
      result_set.map { |record| instantiate_instance_of(self, record, column_types, &block) }
    end
  end
end

Executes a custom SQL query against your database and returns all the results. The results will be returned as an array, with the requested columns encapsulated as attributes of the model you call this method from. For example, if you call Product.find_by_sql, then the results will be returned in a Product object with the attributes you specified in the SQL query.

If you call a complicated SQL query which spans multiple tables, the columns specified by the SELECT will be attributes of the model, whether or not they are columns of the corresponding table.

The sql parameter is a full SQL query as a string. It will be called as is; there will be no database agnostic conversions performed. This should be a last resort because using database-specific terms will lock you into using that particular database engine, or require you to change your call if you switch engines.

# A simple SQL query spanning multiple tables
Post.find_by_sql "SELECT p.title, c.author FROM posts p, comments c WHERE p.id = c.post_id"
# => [#<Post:0x36bff9c @attributes={"title"=>"Ruby Meetup", "first_name"=>"Quentin"}>, ...]

You can use the same string replacement techniques as you can with ActiveRecord::QueryMethods#where:

Post.find_by_sql ["SELECT title FROM posts WHERE author = ? AND created > ?", author_id, start_date]
Post.find_by_sql ["SELECT body FROM comments WHERE author = :user_id OR approved_by = :user_id", { :user_id => user_id }]