Add group_by to ActiveRecord::FinderMethods, take two

activerecord/CHANGELOG.md

@@ -1,3 +1,15 @@
+*   Add `ActiveRecord::FinderMethods#group_by` method, which collects records
+    into sets, grouped by distinct values for the specified `field`. This patch
+    leverages `ActiveRecord::Relation` to be far more efficient than
+    `Enumerable#group_by` when selecting based on a column name.
+
+    Example:
+
+        User.group_by(:role)
+        # => {"normal" => #<ActiveRecord::Relation [...]>, "admin" => #<ActiveRecord::Relation [...]>}
+
+    *Aidan Feldman*
+
 *   Since the `test_help.rb` in Railties now automatically maintains
     your test schema, the `rake db:test:*` tasks are deprecated. This
     doesn't stop you manually running other tasks on your test database

activerecord/lib/active_record/relation/finder_methods.rb

@@ -217,6 +217,27 @@ def exists?(conditions = :none)
       connection.select_value(relation, "#{name} Exists", relation.bind_values) ? true : false
     end
 
+    # Collect records into sets, grouped by distinct values for the specified +field+.
+    #
+    #   User.select([:id, :name])
+    #   => [#<User id: 1, name: "Oscar">, #<User id: 2, name: "Oscar">, #<User id: 3, name: "Foo">
+    #
+    #   User.group_by(:name)
+    #   => {"Foo" => #<ActiveRecord::Relation [...]>, "Oscar" => #<ActiveRecord::Relation [...]>}
+    def group_by(field = nil, &block)
+      if field.nil? || block_given?
+        super(&block)
+      else
+        result = {}
+        select(field).distinct.each do |item|
+          value = item[field]
+          result[value] = where(field => value)
+        end
+
+        result
+      end
+    end
+
     # This method is called whenever no records are found with either a single
     # id or multiple ids and raises a +ActiveRecord::RecordNotFound+ exception.
     #

activerecord/test/cases/finder_test.rb

@@ -883,6 +883,35 @@ def test_finder_with_offset_string
     assert_nothing_raised(ActiveRecord::StatementInvalid) { Topic.offset("3").to_a }
   end
 
+  def test_group_by_with_no_args
+    assert_kind_of Enumerator, Company.all.group_by
+  end
+
+  def test_group_by_with_symbol
+    groups = Company.all.group_by(:type)
+    assert_kind_of Hash, groups
+
+    keys = groups.keys
+    assert_equal 5, keys.size
+    assert_equal [nil, 'Client', 'DependentFirm', 'ExclusivelyDependentFirm', 'Firm'].to_set, keys.to_set
+
+    assert_equal [2, 3, 5, 10], groups['Client'].map(&:id).sort
+    assert_equal [1, 4], groups['Firm'].map(&:id).sort
+    assert_equal nil, groups['Nonexistent']
+  end
+
+  def test_group_by_with_block
+    groups = Company.all.group_by { |c| c.firm_id }
+
+    group = groups[4]
+    assert_kind_of Array, group
+    assert_equal [5, 10], group.map(&:id).sort
+  end
+
+  def test_group_by_with_no_args
+    assert_kind_of Enumerable, Company.all.group_by
+  end
+
   protected
     def bind(statement, *vars)
       if vars.first.is_a?(Hash)

guides/source/active_record_querying.md

@@ -638,8 +638,10 @@ will return instead a maximum of 5 clients beginning with the 31st. The SQL look
 SELECT * FROM clients LIMIT 5 OFFSET 30
 ```
 
-Group
------
+Grouping
+--------
+
+### `group`
 
 To apply a `GROUP BY` clause to the SQL fired by the finder, you can specify the `group` method on the find.
 
@@ -659,6 +661,25 @@ FROM orders
 GROUP BY date(created_at)
 ```
 
+### `group_by`
+
+Similar to [Ruby's `Enumerable#group_by`](http://ruby-doc.org/core/Enumerable.html#method-i-group_by), Active Record's `group_by` method provides a way to retrieve all records that correspond to each distinct value.
+
+For example, to get the list of users with each role:
+
+```ruby
+Address.group_by(:state)
+# => {"AL" => #<ActiveRecord::Relation [...]>, "AK" => #<ActiveRecord::Relation [...]>, ...}
+```
+
+Under the hood, it will execute the SQL to retrieve the distinct values of the column:
+
+```sql
+# SELECT DISTINCT state FROM addresses
+```
+
+Without loading the full records from the database.
+
 Having
 ------