MONGOID-5390: Mongoid::Contextual::Memory#pluck should return nil values (#5331)

* MONGOID-5390: Mongoid::Contextual::Memory#pluck should return nil values. Conveniently, we can now use ActiveSupport's Array#pluck here, introduced in Rails 5.0.

* Array.wrap not needed

* Docs

* Add release notes

* Update mongoid-8.0.txt

* Update mongoid-8.0.txt

* Comment fix

* Update docs/release-notes/mongoid-8.0.txt

Co-authored-by: Oleg Pudeyev <[email protected]>

* Update mongoid-8.0.txt

* Update mongoid-8.0.txt

* More fixes to readme

* Change splat in doc

* Make "when plucking a mix of empty and non-empty values" its own spec

Co-authored-by: shields <[email protected]>
Co-authored-by: Oleg Pudeyev <[email protected]>

Author: johnnyshields

docs/release-notes/mongoid-8.0.txt

@@ -304,6 +304,61 @@ Mongoid 7 behavior:
   end
 
 
+``#pluck`` on Embedded Criteria Returns ``nil`` Values
+------------------------------------------------
+
+Mongoid 8 fixes a bug where calling ``#pluck`` on a Mongoid::Criteria
+for embedded documents discarded nil values. This behavior was
+inconsistent with both the ``#pluck`` method in ActiveSupport and
+with how ``#pluck`` works when reading documents from the database.
+
+Mongoid 8.0 behavior:
+
+.. code-block:: ruby
+
+  class Address
+    include Mongoid::Document
+
+    embedded_in :mall
+
+    field :street, type: String
+  end
+
+  class Mall
+    include Mongoid::Document
+
+    embeds_many :addresses
+  end
+
+  mall = Mall.create!
+  mall.addresses.create!(street: "Elm Street")
+  mall.addresses.create!(street: nil)
+
+  # Pluck from embedded document criteria
+  mall.addresses.all.pluck(:street)
+    #=> ['Elm Street', nil]
+
+Mongoid 7 behavior, given the same setup:
+
+.. code-block:: ruby
+
+  # Pluck from embedded document criteria
+  mall.addresses.all.pluck(:street)
+    #=> ['Elm Street']
+
+For clarity, the following behavior is unchanged from Mongoid 7 to Mongoid 8.0:
+
+.. code-block:: ruby
+
+  # Pluck from database
+  Mall.all.pluck('addresses.street')
+    #=> [ ['Elm Street', nil] ]
+
+  # Pluck using ActiveSupport Array#pluck
+  mall.addresses.pluck(:street)
+    #=> ['Elm Street', nil]
+
+
 Removed ``:drop_dups`` Option from Indexes
 ------------------------------------------
 

lib/mongoid/contextual/memory.rb

@@ -188,15 +188,16 @@ def limit(value)
         self
       end
 
+      # Pluck the field values in memory.
+      #
+      # @example Get the values in memory.
+      #   context.pluck(:name)
+      #
+      # @param [ String | Symbol ] *fields Field(s) to pluck.
+      #
+      # @return [ Array ] The array of plucked values.
       def pluck(*fields)
-        fields = Array.wrap(fields)
-        documents.map do |doc|
-          if fields.size == 1
-            doc[fields.first]
-          else
-            fields.map { |n| doc[n] }.compact
-          end
-        end.compact
+        documents.pluck(*fields)
       end
 
       # Skips the provided number of documents.

spec/mongoid/contextual/memory_spec.rb

@@ -948,6 +948,27 @@
       end
     end
 
+    context "when plucking a mix of empty and non-empty values" do
+
+      let(:empty_doc) do
+        Address.new(street: nil)
+      end
+
+      let(:criteria) do
+        Address.all.tap do |crit|
+          crit.documents = [ hobrecht, friedel, empty_doc ]
+        end
+      end
+
+      let!(:plucked) do
+        context.pluck(:street)
+      end
+
+      it "returns the values" do
+        expect(plucked).to eq([ "hobrecht", "friedel", nil ])
+      end
+    end
+
     context "when plucking a field that doesnt exist" do
 
       context "when pluck one field" do
@@ -957,7 +978,7 @@
         end
 
         it "returns a empty array" do
-          expect(plucked).to eq([])
+          expect(plucked).to eq([nil, nil])
         end
       end
 
@@ -968,7 +989,7 @@
         end
 
         it "returns a empty array" do
-          expect(plucked).to eq([[], []])
+          expect(plucked).to eq([[nil, nil], [nil, nil]])
         end
       end
     end