Merge pull request #38760 from eugeneius/enumerable_pick

Add Enumerable#pick

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   `Relation#pick` now uses already loaded results instead of making another query.
+
+    *Eugene Kenny*
+
 *   Deprecate using `return`, `break` or `throw` to exit a transaction block
 
     *Dylan Thacker-Smith*

activerecord/lib/active_record/relation/calculations.rb

@@ -181,7 +181,7 @@ module ActiveRecord
     # See also #ids.
     #
     def pluck(*column_names)
-      if loaded? && (column_names.map(&:to_s) - @klass.attribute_names - @klass.attribute_aliases.keys).empty?
+      if loaded? && all_attributes?(column_names)
         return records.pluck(*column_names)
       end
 
@@ -218,6 +218,10 @@ module ActiveRecord
     #   # SELECT people.name, people.email_address FROM people WHERE id = 1 LIMIT 1
     #   # => [ 'David', 'david@loudthinking.com' ]
     def pick(*column_names)
+      if loaded? && all_attributes?(column_names)
+        return records.pick(*column_names)
+      end
+
       limit(1).pluck(*column_names).first
     end
 
@@ -230,6 +234,10 @@ module ActiveRecord
     end
 
     private
+      def all_attributes?(column_names)
+        (column_names.map(&:to_s) - @klass.attribute_names - @klass.attribute_aliases.keys).empty?
+      end
+
       def has_include?(column_name)
         eager_loading? || (includes_values.present? && column_name && column_name != :all)
       end

activerecord/test/cases/associations_test.rb

@@ -264,6 +264,13 @@ class AssociationProxyTest < ActiveRecord::TestCase
     assert_no_queries { david.first_posts.pluck(:title) }
   end
 
+  def test_pick_uses_loaded_target
+    david = authors(:david)
+    assert_equal david.first_posts.pick(:title), david.first_posts.load.pick(:title)
+    assert_predicate david.first_posts, :loaded?
+    assert_no_queries { david.first_posts.pick(:title) }
+  end
+
   def test_reset_unloads_target
     david = authors(:david)
     david.posts.reload

activerecord/test/cases/calculations_test.rb

@@ -924,6 +924,30 @@ class CalculationsTest < ActiveRecord::TestCase
     assert_equal cool_first.color, Minivan.pick(:color)
   end
 
+  def test_pick_loaded_relation
+    companies = Company.order(:id).limit(3).load
+
+    assert_no_queries do
+      assert_equal "37signals", companies.pick(:name)
+    end
+  end
+
+  def test_pick_loaded_relation_multiple_columns
+    companies = Company.order(:id).limit(3).load
+
+    assert_no_queries do
+      assert_equal [1, "37signals"], companies.pick(:id, :name)
+    end
+  end
+
+  def test_pick_loaded_relation_sql_fragment
+    companies = Company.order(:name).limit(3).load
+
+    assert_queries 1 do
+      assert_equal "37signals", companies.pick(Arel.sql("DISTINCT name"))
+    end
+  end
+
   def test_grouped_calculation_with_polymorphic_relation
     part = ShipPart.create!(name: "has trinket")
     part.trinkets.create!

activesupport/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Add `Enumerable#pick` to complement `ActiveRecord::Relation#pick`.
+
+    *Eugene Kenny*
+
 *   [Breaking change] `ActiveSupport::Callbacks#halted_callback_hook` now receive a 2nd argument:
 
     `ActiveSupport::Callbacks#halted_callback_hook` now receive the name of the callback

activesupport/lib/active_support/core_ext/enumerable.rb

@@ -142,7 +142,7 @@ module Enumerable
     excluding(*elements)
   end
 
-  # Convert an enumerable to an array based on the given key.
+  # Extract the given key from each element in the enumerable.
   #
   #   [{ name: "David" }, { name: "Rafael" }, { name: "Aaron" }].pluck(:name)
   #   # => ["David", "Rafael", "Aaron"]
@@ -157,6 +157,23 @@ module Enumerable
     end
   end
 
+  # Extract the given key from the first element in the enumerable.
+  #
+  #   [{ name: "David" }, { name: "Rafael" }, { name: "Aaron" }].pick(:name)
+  #   # => "David"
+  #
+  #   [{ id: 1, name: "David" }, { id: 2, name: "Rafael" }].pick(:id, :name)
+  #   # => [1, "David"]
+  def pick(*keys)
+    return if none?
+
+    if keys.many?
+      keys.map { |key| first[key] }
+    else
+      first[keys.first]
+    end
+  end
+
   # Returns a new +Array+ without the blank items.
   # Uses Object#blank? for determining if an item is blank.
   #

activesupport/test/core_ext/enumerable_test.rb

@@ -241,6 +241,24 @@ class EnumerableTests < ActiveSupport::TestCase
       ExpandedPayment.new(10, 50)
     ])
     assert_equal [[5, 99], [15, 0], [10, 50]], payments.pluck(:dollars, :cents)
+
+    assert_equal [], [].pluck(:price)
+    assert_equal [], [].pluck(:dollars, :cents)
+  end
+
+  def test_pick
+    payments = GenericEnumerable.new([ Payment.new(5), Payment.new(15), Payment.new(10) ])
+    assert_equal 5, payments.pick(:price)
+
+    payments = GenericEnumerable.new([
+      ExpandedPayment.new(5, 99),
+      ExpandedPayment.new(15, 0),
+      ExpandedPayment.new(10, 50)
+    ])
+    assert_equal [5, 99], payments.pick(:dollars, :cents)
+
+    assert_nil [].pick(:price)
+    assert_nil [].pick(:dollars, :cents)
   end
 
   def test_compact_blank

guides/source/active_support_core_extensions.md

@@ -2119,10 +2119,22 @@ NOTE: Defined in `active_support/core_ext/enumerable.rb`.
 
 ### `pluck`
 
-The method `pluck` returns an array based on the given key:
+The method `pluck` extracts the given key from each element:
 
 ```ruby
 [{ name: "David" }, { name: "Rafael" }, { name: "Aaron" }].pluck(:name) # => ["David", "Rafael", "Aaron"]
+[{ id: 1, name: "David" }, { id: 2, name: "Rafael" }].pluck(:id, :name) # => [[1, "David"], [2, "Rafael"]]
+```
+
+NOTE: Defined in `active_support/core_ext/enumerable.rb`.
+
+### `pick`
+
+The method `pick` extracts the given key from the first element:
+
+```ruby
+[{ name: "David" }, { name: "Rafael" }, { name: "Aaron" }].pick(:name) # => "David"
+[{ id: 1, name: "David" }, { id: 2, name: "Rafael" }].pick(:id, :name) # => [1, "David"]
 ```
 
 NOTE: Defined in `active_support/core_ext/enumerable.rb`.