From ba279bcb852f88bfa2f2dab1c2763d3faceb6c46 Mon Sep 17 00:00:00 2001
From: Neil Shweky <neilshweky@gmail.com>
Date: Mon, 13 Jul 2020 08:25:58 -0400
Subject: [PATCH 01/10] MONGOID-4921 add discriminator_key as a class variable

---
 lib/mongoid/traversable.rb        | 24 ++++++++-
 spec/mongoid/traversable_spec.rb  | 83 +++++++++++++++++++++++++++++++
 spec/support/models/guitar.rb     |  5 ++
 spec/support/models/instrument.rb | 10 ++++
 spec/support/models/piano.rb      |  5 ++
 5 files changed, 125 insertions(+), 2 deletions(-)
 create mode 100644 spec/support/models/guitar.rb
 create mode 100644 spec/support/models/instrument.rb
 create mode 100644 spec/support/models/piano.rb

diff --git a/lib/mongoid/traversable.rb b/lib/mongoid/traversable.rb
index f0042987142..ab4e3d63b3e 100644
--- a/lib/mongoid/traversable.rb
+++ b/lib/mongoid/traversable.rb
@@ -2,6 +2,10 @@
 # encoding: utf-8
 
 module Mongoid
+  extend self
+
+  attr_accessor :discriminator_key
+  self.discriminator_key = '_type'
 
   # Provides behavior around traversing the document graph.
   #
@@ -17,6 +21,21 @@ def _parent=(p)
       @__parent = p
     end
 
+    included do 
+      def self.discriminator_key
+        @discriminator_key || Mongoid.discriminator_key
+      end
+
+      def self.discriminator_key=(value) 
+        @discriminator_key = value
+        descendants.each do |child| 
+          child.discriminator_key = value
+        end
+      end
+
+      
+    end
+
     # Get all child +Documents+ to this +Document+, going n levels deep if
     # necessary. This is used when calling update persistence operations from
     # the root document, where changes in the entire tree need to be
@@ -205,10 +224,11 @@ def inherited(subclass)
         # We only need the _type field if inheritance is in play, but need to
         # add to the root class as well for backwards compatibility.
         unless fields.has_key?("_type")
-          field(:_type, default: self.name, type: String)
+          field(self.discriminator_key, default: self.name, type: String)
         end
+
         subclass_default = subclass.name || ->{ self.class.name }
-        subclass.field(:_type, default: subclass_default, type: String, overwrite: true)
+        subclass.field(self.discriminator_key, default: subclass_default, type: String, overwrite: true)
       end
     end
   end
diff --git a/spec/mongoid/traversable_spec.rb b/spec/mongoid/traversable_spec.rb
index 0004cff9361..42e73e9f7f9 100644
--- a/spec/mongoid/traversable_spec.rb
+++ b/spec/mongoid/traversable_spec.rb
@@ -251,4 +251,87 @@
       end
     end
   end
+
+  describe "#discriminator_key" do
+
+    context "when the discriminator key is not changed" do 
+      it "equals _type" do
+        expect(Instrument.discriminator_key).to eq("_type")
+      end
+
+      it "children's discriminator keys equal _type" do
+        expect(Guitar.discriminator_key).to eq("_type")
+        expect(Piano.discriminator_key).to eq("_type")
+      end
+
+      it "the base discriminator key is _type" do
+        expect(Mongoid.discriminator_key).to eq("_type")
+      end
+    end
+
+    context "when the discriminator key is changed at the base level" do 
+      before do
+        Mongoid.discriminator_key = "hello"
+      end
+
+      after do
+        Mongoid.discriminator_key = "_type"
+      end
+
+      it "the value changes" do 
+        expect(Mongoid.discriminator_key).to eq("hello")
+      end
+
+      it "all discriminator keys change" do 
+        expect(Instrument.discriminator_key).to eq("hello")
+        expect(Guitar.discriminator_key).to eq("hello")
+        expect(Piano.discriminator_key).to eq("hello")
+      end
+    end
+
+    context "when the discriminator key is changed in the parent" do 
+      before do
+        Instrument.discriminator_key = "hello2"
+      end
+
+      after do 
+        Instrument.discriminator_key = "_type"
+      end
+
+      it "changes in the child classes" do 
+        expect(Guitar.discriminator_key).to eq("hello2")
+        expect(Piano.discriminator_key).to eq("hello2")
+      end
+
+      it "the base discriminator key is _type" do
+        expect(Mongoid.discriminator_key).to eq("_type")
+      end
+    end 
+
+    context "when the discriminator key is changed in the child" do 
+      before do
+        Guitar.discriminator_key = "hello3"
+      end
+
+      after do 
+        Piano.discriminator_key = "_type"
+      end
+
+      it "changes in the child classes" do 
+        expect(Guitar.discriminator_key).to eq("hello3")
+      end
+
+      it "doesn't change in the sibling" do 
+        expect(Piano.discriminator_key).to eq("_type")
+      end
+
+      it "doesn't change in the parent" do 
+        expect(Instrument.discriminator_key).to eq("_type")
+      end
+
+      it "the base discriminator key is _type" do
+        expect(Mongoid.discriminator_key).to eq("_type")
+      end
+    end
+  end
 end
diff --git a/spec/support/models/guitar.rb b/spec/support/models/guitar.rb
new file mode 100644
index 00000000000..56e66fe1b65
--- /dev/null
+++ b/spec/support/models/guitar.rb
@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+# encoding: utf-8
+
+class Guitar < Instrument
+end
\ No newline at end of file
diff --git a/spec/support/models/instrument.rb b/spec/support/models/instrument.rb
new file mode 100644
index 00000000000..c8092fdb515
--- /dev/null
+++ b/spec/support/models/instrument.rb
@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+# encoding: utf-8
+
+class Instrument
+    include Mongoid::Document
+    field :owner, type: String, default: 0
+end
+
+require "support/models/guitar"
+require "support/models/piano"
\ No newline at end of file
diff --git a/spec/support/models/piano.rb b/spec/support/models/piano.rb
new file mode 100644
index 00000000000..ec094b1a59c
--- /dev/null
+++ b/spec/support/models/piano.rb
@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+# encoding: utf-8
+
+class Piano < Instrument
+end
\ No newline at end of file

From 35d782ecc28599132ac9490f005c58df6c06349e Mon Sep 17 00:00:00 2001
From: Neil Shweky <neilshweky@gmail.com>
Date: Mon, 13 Jul 2020 08:28:37 -0400
Subject: [PATCH 02/10] MONGOID-4921 clean up code

---
 lib/mongoid/traversable.rb        | 2 --
 spec/support/models/guitar.rb     | 2 +-
 spec/support/models/instrument.rb | 2 +-
 spec/support/models/piano.rb      | 2 +-
 4 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/lib/mongoid/traversable.rb b/lib/mongoid/traversable.rb
index ab4e3d63b3e..c148eac41dd 100644
--- a/lib/mongoid/traversable.rb
+++ b/lib/mongoid/traversable.rb
@@ -32,8 +32,6 @@ def self.discriminator_key=(value)
           child.discriminator_key = value
         end
       end
-
-      
     end
 
     # Get all child +Documents+ to this +Document+, going n levels deep if
diff --git a/spec/support/models/guitar.rb b/spec/support/models/guitar.rb
index 56e66fe1b65..689cc9b9775 100644
--- a/spec/support/models/guitar.rb
+++ b/spec/support/models/guitar.rb
@@ -2,4 +2,4 @@
 # encoding: utf-8
 
 class Guitar < Instrument
-end
\ No newline at end of file
+end
diff --git a/spec/support/models/instrument.rb b/spec/support/models/instrument.rb
index c8092fdb515..55f95b340c6 100644
--- a/spec/support/models/instrument.rb
+++ b/spec/support/models/instrument.rb
@@ -7,4 +7,4 @@ class Instrument
 end
 
 require "support/models/guitar"
-require "support/models/piano"
\ No newline at end of file
+require "support/models/piano"
diff --git a/spec/support/models/piano.rb b/spec/support/models/piano.rb
index ec094b1a59c..af48420456a 100644
--- a/spec/support/models/piano.rb
+++ b/spec/support/models/piano.rb
@@ -2,4 +2,4 @@
 # encoding: utf-8
 
 class Piano < Instrument
-end
\ No newline at end of file
+end

From 48bf99089ea333f3d133d8efe253fe2e4578a361 Mon Sep 17 00:00:00 2001
From: Neil Shweky <neilshweky@gmail.com>
Date: Mon, 13 Jul 2020 10:45:54 -0400
Subject: [PATCH 03/10] MONGOID-4921 add tests, docs

---
 lib/mongoid.rb                   | 11 +++++++++++
 lib/mongoid/traversable.rb       |  8 ++++----
 spec/mongoid/traversable_spec.rb |  4 ++--
 spec/support/models/guitar.rb    |  1 +
 4 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/lib/mongoid.rb b/lib/mongoid.rb
index fab4daf08e1..8b5bc4ffbf4 100644
--- a/lib/mongoid.rb
+++ b/lib/mongoid.rb
@@ -26,6 +26,7 @@
 require "mongoid/tasks/database"
 require "mongoid/query_cache"
 
+
 # If we are using Rails then we will include the Mongoid railtie. This has all
 # the nifty initializers that Mongoid needs.
 if defined?(Rails)
@@ -111,4 +112,14 @@ def client(name)
   #
   # @since 1.0.0
   def_delegators Config, *(Config.public_instance_methods(false) - [ :logger=, :logger ])
+
+  # The default discriminator key for Mongoid
+  #
+  # @example get the base discriminator key
+  #   Mongoid.discriminator_key
+  #
+  # @example set the base discriminator key
+  #   Mongoid.discriminator_key = "test"
+  attr_accessor :discriminator_key
+  self.discriminator_key = '_type'
 end
diff --git a/lib/mongoid/traversable.rb b/lib/mongoid/traversable.rb
index c148eac41dd..7c9b68f1379 100644
--- a/lib/mongoid/traversable.rb
+++ b/lib/mongoid/traversable.rb
@@ -2,10 +2,6 @@
 # encoding: utf-8
 
 module Mongoid
-  extend self
-
-  attr_accessor :discriminator_key
-  self.discriminator_key = '_type'
 
   # Provides behavior around traversing the document graph.
   #
@@ -28,6 +24,10 @@ def self.discriminator_key
 
       def self.discriminator_key=(value) 
         @discriminator_key = value
+        # unless fields.has_key?(@discriminator_key)
+        #   field(@discriminator_key, default: self.name, type: String)
+        # end 
+
         descendants.each do |child| 
           child.discriminator_key = value
         end
diff --git a/spec/mongoid/traversable_spec.rb b/spec/mongoid/traversable_spec.rb
index 42e73e9f7f9..3a1d9668501 100644
--- a/spec/mongoid/traversable_spec.rb
+++ b/spec/mongoid/traversable_spec.rb
@@ -260,7 +260,7 @@
       end
 
       it "children's discriminator keys equal _type" do
-        expect(Guitar.discriminator_key).to eq("_type")
+        expect(Guitar.discriminator_key).to eq("dkey")
         expect(Piano.discriminator_key).to eq("_type")
       end
 
@@ -284,7 +284,7 @@
 
       it "all discriminator keys change" do 
         expect(Instrument.discriminator_key).to eq("hello")
-        expect(Guitar.discriminator_key).to eq("hello")
+        expect(Guitar.discriminator_key).to eq("dkey")
         expect(Piano.discriminator_key).to eq("hello")
       end
     end
diff --git a/spec/support/models/guitar.rb b/spec/support/models/guitar.rb
index 689cc9b9775..0a061ada35a 100644
--- a/spec/support/models/guitar.rb
+++ b/spec/support/models/guitar.rb
@@ -2,4 +2,5 @@
 # encoding: utf-8
 
 class Guitar < Instrument
+    self.discriminator_key = "dkey"
 end

From 063957813fb80c7d636d4ec9e9b4863494768c23 Mon Sep 17 00:00:00 2001
From: Neil Shweky <neilshweky@gmail.com>
Date: Mon, 13 Jul 2020 10:47:14 -0400
Subject: [PATCH 04/10] MONGOID-4921 fix whitespace

---
 lib/mongoid.rb | 1 -
 1 file changed, 1 deletion(-)

diff --git a/lib/mongoid.rb b/lib/mongoid.rb
index 8b5bc4ffbf4..407cd70ba0a 100644
--- a/lib/mongoid.rb
+++ b/lib/mongoid.rb
@@ -26,7 +26,6 @@
 require "mongoid/tasks/database"
 require "mongoid/query_cache"
 
-
 # If we are using Rails then we will include the Mongoid railtie. This has all
 # the nifty initializers that Mongoid needs.
 if defined?(Rails)

From 6ed2d6144d3edbf8e02bb5ef283968d80db10d19 Mon Sep 17 00:00:00 2001
From: Neil Shweky <neilshweky@gmail.com>
Date: Mon, 13 Jul 2020 12:24:38 -0400
Subject: [PATCH 05/10] MONGOID-4921 update instrument class

---
 spec/support/models/instrument.rb | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/spec/support/models/instrument.rb b/spec/support/models/instrument.rb
index 55f95b340c6..3b0876b4f29 100644
--- a/spec/support/models/instrument.rb
+++ b/spec/support/models/instrument.rb
@@ -3,7 +3,7 @@
 
 class Instrument
     include Mongoid::Document
-    field :owner, type: String, default: 0
+    field :owner, type: String
 end
 
 require "support/models/guitar"

From dd8586273e3bd27d33c66b92290f39120567e6f8 Mon Sep 17 00:00:00 2001
From: Neil Shweky <neilshweky@gmail.com>
Date: Mon, 13 Jul 2020 19:57:18 -0400
Subject: [PATCH 06/10] MONGOID-4921 clean up models; added error; cleaned
 tests; fixed prepend

---
 lib/config/locales/en.yml                     |  7 +++
 lib/mongoid/errors.rb                         |  1 +
 .../setting_discriminator_key_on_child.rb     | 24 ++++++++
 lib/mongoid/traversable.rb                    | 40 +++++++++----
 spec/mongoid/traversable_spec.rb              | 57 +++++++++----------
 spec/support/models/guitar.rb                 |  1 -
 spec/support/models/instrument.rb             |  1 -
 7 files changed, 88 insertions(+), 43 deletions(-)
 create mode 100644 lib/mongoid/errors/setting_discriminator_key_on_child.rb

diff --git a/lib/config/locales/en.yml b/lib/config/locales/en.yml
index 7399bd590b8..83fd8b5df00 100644
--- a/lib/config/locales/en.yml
+++ b/lib/config/locales/en.yml
@@ -502,6 +502,13 @@ en:
             server versions 3.6 and higher."
           resolution: "Verify that all servers in your deployment are at least
             version 3.6 or don't attempt to use sessions with older server versions."
+        setting_discriminator_key_on_child:
+          message: "Setting the discriminator key on a child class in not allowed."
+          summary: "The discriminator key on the class %{class_name} was attempted
+            to be changed. Changing the discriminator key on a child class is not
+            allowed."
+          resolution: "Try setting the discriminator key on %{superclass} or one of %{class_name}'s
+            ancestors."
         taken:
           "is already taken"
         too_many_nested_attribute_records:
diff --git a/lib/mongoid/errors.rb b/lib/mongoid/errors.rb
index 7e7a52f608b..2399e258e76 100644
--- a/lib/mongoid/errors.rb
+++ b/lib/mongoid/errors.rb
@@ -48,6 +48,7 @@
 require "mongoid/errors/readonly_attribute"
 require "mongoid/errors/readonly_document"
 require "mongoid/errors/scope_overwrite"
+require "mongoid/errors/setting_discriminator_key_on_child"
 require "mongoid/errors/too_many_nested_attribute_records"
 require "mongoid/errors/unknown_attribute"
 require "mongoid/errors/unknown_model"
diff --git a/lib/mongoid/errors/setting_discriminator_key_on_child.rb b/lib/mongoid/errors/setting_discriminator_key_on_child.rb
new file mode 100644
index 00000000000..c1af0673d3d
--- /dev/null
+++ b/lib/mongoid/errors/setting_discriminator_key_on_child.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+# encoding: utf-8
+
+module Mongoid
+    module Errors
+  
+      # This error is raised when trying to create a scope with an name already
+      # taken by another scope or method
+      #
+      # @example Create the error.
+      #   ScopeOverwrite.new(Person,'teenies')
+      class SettingDiscriminatorKeyOnChild < MongoidError
+        def initialize(class_name, superclass)
+          super(
+            compose_message(
+              "setting_discriminator_key_on_child",
+              { class_name: class_name,  superclass: superclass}
+            )
+          )
+        end
+      end
+    end
+  end
+  
\ No newline at end of file
diff --git a/lib/mongoid/traversable.rb b/lib/mongoid/traversable.rb
index 7c9b68f1379..07ba7a76374 100644
--- a/lib/mongoid/traversable.rb
+++ b/lib/mongoid/traversable.rb
@@ -18,20 +18,36 @@ def _parent=(p)
     end
 
     included do 
-      def self.discriminator_key
-        @discriminator_key || Mongoid.discriminator_key
-      end
 
-      def self.discriminator_key=(value) 
-        @discriminator_key = value
-        # unless fields.has_key?(@discriminator_key)
-        #   field(@discriminator_key, default: self.name, type: String)
-        # end 
+      module DiscriminatorKeyAssignment
+        def discriminator_key=(value)
+          if hereditary?
+            raise Errors::SettingDiscriminatorKeyOnChild.new(self.to_s, self.superclass.to_s)
+          end
 
-        descendants.each do |child| 
-          child.discriminator_key = value
+          value = Mongoid.discriminator_key unless value
+          super
         end
       end
+
+      class_attribute :discriminator_key, instance_accessor: false
+      class << self
+        delegate :discriminator_key, to: ::Mongoid
+        prepend DiscriminatorKeyAssignment
+      end
+
+      # def self.discriminator_key
+      #   return @discriminator_key if @discriminator_key
+      #   return superclass.discriminator_key if superclass.respond_to?(:discriminator_key)
+      #   Mongoid.discriminator_key
+      # end
+
+      # def self.discriminator_key=(value)
+      #   if self.hereditary?
+      #     raise NoMethodError.new "You can only set the discriminator key in the parent"
+      #   end
+      #   @discriminator_key = value
+      # end
     end
 
     # Get all child +Documents+ to this +Document+, going n levels deep if
@@ -222,11 +238,11 @@ def inherited(subclass)
         # We only need the _type field if inheritance is in play, but need to
         # add to the root class as well for backwards compatibility.
         unless fields.has_key?("_type")
-          field(self.discriminator_key, default: self.name, type: String)
+          field(:_type, default: self.name, type: String)
         end
 
         subclass_default = subclass.name || ->{ self.class.name }
-        subclass.field(self.discriminator_key, default: subclass_default, type: String, overwrite: true)
+        subclass.field(:_type, default: subclass_default, type: String, overwrite: true)
       end
     end
   end
diff --git a/spec/mongoid/traversable_spec.rb b/spec/mongoid/traversable_spec.rb
index 3a1d9668501..35d2d0a5fe6 100644
--- a/spec/mongoid/traversable_spec.rb
+++ b/spec/mongoid/traversable_spec.rb
@@ -259,17 +259,17 @@
         expect(Instrument.discriminator_key).to eq("_type")
       end
 
-      it "children's discriminator keys equal _type" do
-        expect(Guitar.discriminator_key).to eq("dkey")
-        expect(Piano.discriminator_key).to eq("_type")
+      it "the global discriminator key is _type" do
+        expect(Mongoid.discriminator_key).to eq("_type")
       end
 
-      it "the base discriminator key is _type" do
-        expect(Mongoid.discriminator_key).to eq("_type")
+      it "child discriminator keys equal _type" do
+        expect(Piano.discriminator_key).to eq("_type")
+        expect(Guitar.discriminator_key).to eq("_type")
       end
     end
-
-    context "when the discriminator key is changed at the base level" do 
+    
+    context "when the discriminator key is changed at the global level" do 
       before do
         Mongoid.discriminator_key = "hello"
       end
@@ -284,8 +284,8 @@
 
       it "all discriminator keys change" do 
         expect(Instrument.discriminator_key).to eq("hello")
-        expect(Guitar.discriminator_key).to eq("dkey")
         expect(Piano.discriminator_key).to eq("hello")
+        expect(Guitar.discrdiminator_key).to eq("hello")
       end
     end
 
@@ -295,32 +295,35 @@
       end
 
       after do 
-        Instrument.discriminator_key = "_type"
+        Instrument.discriminator_key = nil
       end
 
-      it "changes in the child classes" do 
-        expect(Guitar.discriminator_key).to eq("hello2")
-        expect(Piano.discriminator_key).to eq("hello2")
+      it "the global discriminator key is _type" do
+        expect(Mongoid.discriminator_key).to eq("_type")
       end
 
-      it "the base discriminator key is _type" do
-        expect(Mongoid.discriminator_key).to eq("_type")
+      it "changes in the child classes" do 
+        expect(Piano.discriminator_key).to eq("hello2")
+        expect(Guitar.discriminator_key).to eq("hello2")
       end
     end 
 
     context "when the discriminator key is changed in the child" do 
-      before do
-        Guitar.discriminator_key = "hello3"
-      end
-
-      after do 
-        Piano.discriminator_key = "_type"
-      end
-
-      it "changes in the child classes" do 
-        expect(Guitar.discriminator_key).to eq("hello3")
+      
+      it "raises an error" do
+        expect do
+          Guitar.discriminator_key = "hello3"
+        end.to raise_error(Mongoid::Errors::SettingDiscriminatorKeyOnChild)
+      end
+      
+      it "doesn't change in that class" do 
+        expect(Guitar.discriminator_key).to eq("_type")
+      end
+      
+      it "the global discriminator key is _type" do
+        expect(Mongoid.discriminator_key).to eq("_type")
       end
-
+      
       it "doesn't change in the sibling" do 
         expect(Piano.discriminator_key).to eq("_type")
       end
@@ -328,10 +331,6 @@
       it "doesn't change in the parent" do 
         expect(Instrument.discriminator_key).to eq("_type")
       end
-
-      it "the base discriminator key is _type" do
-        expect(Mongoid.discriminator_key).to eq("_type")
-      end
     end
   end
 end
diff --git a/spec/support/models/guitar.rb b/spec/support/models/guitar.rb
index 0a061ada35a..689cc9b9775 100644
--- a/spec/support/models/guitar.rb
+++ b/spec/support/models/guitar.rb
@@ -2,5 +2,4 @@
 # encoding: utf-8
 
 class Guitar < Instrument
-    self.discriminator_key = "dkey"
 end
diff --git a/spec/support/models/instrument.rb b/spec/support/models/instrument.rb
index 3b0876b4f29..e3c96db07f6 100644
--- a/spec/support/models/instrument.rb
+++ b/spec/support/models/instrument.rb
@@ -3,7 +3,6 @@
 
 class Instrument
     include Mongoid::Document
-    field :owner, type: String
 end
 
 require "support/models/guitar"

From 1390691121779e53bd8022456aed5d29ecaa9455 Mon Sep 17 00:00:00 2001
From: Neil Shweky <neilshweky@gmail.com>
Date: Mon, 13 Jul 2020 20:23:58 -0400
Subject: [PATCH 07/10] MONGOID-4921 add global discriminator key as config
 option

---
 lib/mongoid.rb              | 10 ----------
 lib/mongoid/config.rb       |  3 +++
 lib/mongoid/serializable.rb |  2 ++
 lib/mongoid/traversable.rb  | 14 --------------
 spec/mongoid/config_spec.rb | 28 ++++++++++++++++++++++++++++
 5 files changed, 33 insertions(+), 24 deletions(-)

diff --git a/lib/mongoid.rb b/lib/mongoid.rb
index 407cd70ba0a..fab4daf08e1 100644
--- a/lib/mongoid.rb
+++ b/lib/mongoid.rb
@@ -111,14 +111,4 @@ def client(name)
   #
   # @since 1.0.0
   def_delegators Config, *(Config.public_instance_methods(false) - [ :logger=, :logger ])
-
-  # The default discriminator key for Mongoid
-  #
-  # @example get the base discriminator key
-  #   Mongoid.discriminator_key
-  #
-  # @example set the base discriminator key
-  #   Mongoid.discriminator_key = "test"
-  attr_accessor :discriminator_key
-  self.discriminator_key = '_type'
 end
diff --git a/lib/mongoid/config.rb b/lib/mongoid/config.rb
index 557a6518b36..e4e1bb2170e 100644
--- a/lib/mongoid/config.rb
+++ b/lib/mongoid/config.rb
@@ -32,6 +32,9 @@ module Config
     # error.
     option :belongs_to_required_by_default, default: true
 
+    # Set the global discriminator key.
+    option :discriminator_key, default: "_type"
+
     # Raise an exception when a field is redefined.
     option :duplicate_fields_exception, default: false
 
diff --git a/lib/mongoid/serializable.rb b/lib/mongoid/serializable.rb
index 245c9d8a94f..eeaabc0b4aa 100644
--- a/lib/mongoid/serializable.rb
+++ b/lib/mongoid/serializable.rb
@@ -22,6 +22,8 @@ class << self
         # functionality and not the Ruby standard library one.
         # See https://jira.mongodb.org/browse/MONGOID-4849.
         delegate :include_root_in_json, to: ::Mongoid
+        delegate :discriminator_key, to: ::Mongoid
+
       end
     end
 
diff --git a/lib/mongoid/traversable.rb b/lib/mongoid/traversable.rb
index 07ba7a76374..2e640efc3c9 100644
--- a/lib/mongoid/traversable.rb
+++ b/lib/mongoid/traversable.rb
@@ -18,7 +18,6 @@ def _parent=(p)
     end
 
     included do 
-
       module DiscriminatorKeyAssignment
         def discriminator_key=(value)
           if hereditary?
@@ -35,19 +34,6 @@ class << self
         delegate :discriminator_key, to: ::Mongoid
         prepend DiscriminatorKeyAssignment
       end
-
-      # def self.discriminator_key
-      #   return @discriminator_key if @discriminator_key
-      #   return superclass.discriminator_key if superclass.respond_to?(:discriminator_key)
-      #   Mongoid.discriminator_key
-      # end
-
-      # def self.discriminator_key=(value)
-      #   if self.hereditary?
-      #     raise NoMethodError.new "You can only set the discriminator key in the parent"
-      #   end
-      #   @discriminator_key = value
-      # end
     end
 
     # Get all child +Documents+ to this +Document+, going n levels deep if
diff --git a/spec/mongoid/config_spec.rb b/spec/mongoid/config_spec.rb
index 9d6c288d177..f40f4ad6611 100644
--- a/spec/mongoid/config_spec.rb
+++ b/spec/mongoid/config_spec.rb
@@ -197,6 +197,34 @@
     end
   end
 
+  context 'when discriminator_key option' do
+    context 'is not set in the config' do
+      it 'sets the value to _type by default' do
+        Mongoid::Config.reset
+        configuration = CONFIG.merge(options: {})
+
+        Mongoid.configure { |config| config.load_configuration(configuration) }
+
+        expect(Mongoid::Config.discriminator_key).to be("_type")
+      end
+    end
+
+    context 'is set in the config' do
+      it 'sets the value' do
+        Mongoid::Config.reset
+        configuration = CONFIG.merge(options: {discriminator_key: "test"})
+
+        Mongoid.configure { |config| config.load_configuration(configuration) }
+
+        expect(Mongoid::Config.discriminator_key).to be("test")
+      end
+
+      it 'is set globally' do 
+        expect(Mongoid.discriminator_key).to be("test")
+      end
+    end
+  end
+
   describe "#load!" do
 
     before(:all) do

From 99775ecda3d36826f01534446d2e2c595a83e560 Mon Sep 17 00:00:00 2001
From: Neil Shweky <neilshweky@gmail.com>
Date: Mon, 13 Jul 2020 20:25:49 -0400
Subject: [PATCH 08/10] MONGOID-4921 add discriminator key to the config docs

---
 docs/tutorials/mongoid-configuration.txt | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/docs/tutorials/mongoid-configuration.txt b/docs/tutorials/mongoid-configuration.txt
index d902304b54f..a4a7f625704 100644
--- a/docs/tutorials/mongoid-configuration.txt
+++ b/docs/tutorials/mongoid-configuration.txt
@@ -245,6 +245,9 @@ can be configured.
       # error. (default: true)
       belongs_to_required_by_default: true
       
+      # Set the global discriminator key. (default: "_type")
+      discriminator: "_type"
+
       # Raise an exception when a field is redefined. (default: false)
       duplicate_fields_exception: false
       

From 7da5e7fba851d074dfb1bf8b78ebdb9985624dfb Mon Sep 17 00:00:00 2001
From: Neil Shweky <neilshweky@gmail.com>
Date: Mon, 13 Jul 2020 20:30:35 -0400
Subject: [PATCH 09/10] MONGOID-4921 fixed typos, cleaned up code

---
 docs/tutorials/mongoid-configuration.txt      |  2 +-
 lib/config/locales/en.yml                     |  2 +-
 .../setting_discriminator_key_on_child.rb     | 31 +++++++++----------
 lib/mongoid/serializable.rb                   |  2 --
 4 files changed, 17 insertions(+), 20 deletions(-)

diff --git a/docs/tutorials/mongoid-configuration.txt b/docs/tutorials/mongoid-configuration.txt
index a4a7f625704..3a2d89ecb34 100644
--- a/docs/tutorials/mongoid-configuration.txt
+++ b/docs/tutorials/mongoid-configuration.txt
@@ -246,7 +246,7 @@ can be configured.
       belongs_to_required_by_default: true
       
       # Set the global discriminator key. (default: "_type")
-      discriminator: "_type"
+      discriminator_key: "_type"
 
       # Raise an exception when a field is redefined. (default: false)
       duplicate_fields_exception: false
diff --git a/lib/config/locales/en.yml b/lib/config/locales/en.yml
index 83fd8b5df00..8e1e168fe6f 100644
--- a/lib/config/locales/en.yml
+++ b/lib/config/locales/en.yml
@@ -503,7 +503,7 @@ en:
           resolution: "Verify that all servers in your deployment are at least
             version 3.6 or don't attempt to use sessions with older server versions."
         setting_discriminator_key_on_child:
-          message: "Setting the discriminator key on a child class in not allowed."
+          message: "Setting the discriminator key on a child class is not allowed."
           summary: "The discriminator key on the class %{class_name} was attempted
             to be changed. Changing the discriminator key on a child class is not
             allowed."
diff --git a/lib/mongoid/errors/setting_discriminator_key_on_child.rb b/lib/mongoid/errors/setting_discriminator_key_on_child.rb
index c1af0673d3d..8183debd063 100644
--- a/lib/mongoid/errors/setting_discriminator_key_on_child.rb
+++ b/lib/mongoid/errors/setting_discriminator_key_on_child.rb
@@ -2,23 +2,22 @@
 # encoding: utf-8
 
 module Mongoid
-    module Errors
-  
-      # This error is raised when trying to create a scope with an name already
-      # taken by another scope or method
-      #
-      # @example Create the error.
-      #   ScopeOverwrite.new(Person,'teenies')
-      class SettingDiscriminatorKeyOnChild < MongoidError
-        def initialize(class_name, superclass)
-          super(
-            compose_message(
-              "setting_discriminator_key_on_child",
-              { class_name: class_name,  superclass: superclass}
-            )
+  module Errors
+
+    # This error is raised when trying to create a scope with an name already
+    # taken by another scope or method
+    #
+    # @example Create the error.
+    #   ScopeOverwrite.new(Person,'teenies')
+    class SettingDiscriminatorKeyOnChild < MongoidError
+      def initialize(class_name, superclass)
+        super(
+          compose_message(
+            "setting_discriminator_key_on_child",
+            { class_name: class_name,  superclass: superclass}
           )
-        end
+        )
       end
     end
   end
-  
\ No newline at end of file
+end
diff --git a/lib/mongoid/serializable.rb b/lib/mongoid/serializable.rb
index eeaabc0b4aa..245c9d8a94f 100644
--- a/lib/mongoid/serializable.rb
+++ b/lib/mongoid/serializable.rb
@@ -22,8 +22,6 @@ class << self
         # functionality and not the Ruby standard library one.
         # See https://jira.mongodb.org/browse/MONGOID-4849.
         delegate :include_root_in_json, to: ::Mongoid
-        delegate :discriminator_key, to: ::Mongoid
-
       end
     end
 

From db017b53ac54740822fe78794c89021c7cbfec5b Mon Sep 17 00:00:00 2001
From: Neil Shweky <neilshweky@gmail.com>
Date: Mon, 13 Jul 2020 20:37:30 -0400
Subject: [PATCH 10/10] MONGOID-4921 add test for calling discriminator_key on
 instance

---
 spec/mongoid/traversable_spec.rb | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/spec/mongoid/traversable_spec.rb b/spec/mongoid/traversable_spec.rb
index 35d2d0a5fe6..e690157ad3f 100644
--- a/spec/mongoid/traversable_spec.rb
+++ b/spec/mongoid/traversable_spec.rb
@@ -255,6 +255,7 @@
   describe "#discriminator_key" do
 
     context "when the discriminator key is not changed" do 
+
       it "equals _type" do
         expect(Instrument.discriminator_key).to eq("_type")
       end
@@ -270,6 +271,7 @@
     end
     
     context "when the discriminator key is changed at the global level" do 
+
       before do
         Mongoid.discriminator_key = "hello"
       end
@@ -285,11 +287,12 @@
       it "all discriminator keys change" do 
         expect(Instrument.discriminator_key).to eq("hello")
         expect(Piano.discriminator_key).to eq("hello")
-        expect(Guitar.discrdiminator_key).to eq("hello")
+        expect(Guitar.discriminator_key).to eq("hello")
       end
     end
 
     context "when the discriminator key is changed in the parent" do 
+
       before do
         Instrument.discriminator_key = "hello2"
       end
@@ -309,7 +312,7 @@
     end 
 
     context "when the discriminator key is changed in the child" do 
-      
+
       it "raises an error" do
         expect do
           Guitar.discriminator_key = "hello3"
@@ -332,5 +335,24 @@
         expect(Instrument.discriminator_key).to eq("_type")
       end
     end
+
+    context "when discriminator key is called on an instance" do 
+
+      let(:guitar) do
+        Guitar.new
+      end
+
+      it "raises an error on setter" do
+        expect do
+          guitar.discriminator_key = "hello3"
+        end.to raise_error(NoMethodError)
+      end
+
+      it "raises an error on getter" do
+        expect do
+          guitar.discriminator_key
+        end.to raise_error(NoMethodError)
+      end
+    end
   end
 end