Add group_by to ActiveRecord::FinderMethods

activerecord/CHANGELOG.md

@@ -1,3 +1,12 @@
+*   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*
+
 *   Handle aliased attributes in ActiveRecord::Relation.
 
     When using symbol keys, ActiveRecord will now translate aliased attribute names to the actual column name used in the database:

activerecord/lib/active_record/relation/finder_methods.rb

@@ -176,6 +176,28 @@ def exists?(conditions = :none)
       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(*args, &block)
+      field = args[0]
+      if field.nil? || block_given?
+        super(&block)
+      else
+        result = {}
+        self.select(field).distinct.each do |item|
+          value = item[field]
+          result[value] = self.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

@@ -849,6 +849,31 @@ def test_finder_with_offset_string
     assert_nothing_raised(ActiveRecord::StatementInvalid) { Topic.all.merge!(:offset => "3").to_a }
   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 2, group.size
+  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)