MONGOID-5270 Implement .tally method for Mongoid#Criteria (#5344)

* MONGOID-5270 initial implementation of tally

* MONGOID-5270 finish and fully test tally feature

* MONGOID-5270 add typeless field test

* MONGOID-5270 don't change existing behavior

* MONGOID-5270 correct documentation

* MONGOID-5702 docs, release note, renaming

* MONGOID-5270 add none case

* MONGOID-5270 tweaks to pluck and tally logic to make them function correctly and consistently

* MONGOID-5270 update comment

* MONGOID-5270 add tally implementation

* MONGOID-5270 change to not use tally

* MONGOID-5270 add duplicate keys test

* MONGOID-5270 move fetch_and_demongoize and change indifferent access

* MONGOID-5270 use eq instead of be

* Revert "MONGOID-5270 use eq instead of be"

This reverts commit 25d65effdb711fd61737a35610106ddc2723671c.

Author: neilshweky

source/reference/queries.txt

@@ -1380,6 +1380,27 @@ Mongoid also has some helpful methods on criteria.
           # expands out to "managers.name" in the query:
           Band.all.pluck('managers.n')
 
+   * - ``Criteria#tally``
+
+       *Get a mapping of values to counts for the provided field.*
+
+       *This method accepts the dot notation, thus permitting referencing
+       fields in embedded associations.*
+
+       *This method respects :ref:`field aliases <field-aliases>`,
+       including those defined in embedded documents.*
+
+     -
+        .. code-block:: ruby
+
+          Band.all.tally(:name)
+
+          Band.all.tally('cities.name')
+
+          # Using the earlier definition of Manager,
+          # expands out to "managers.name" in the query:
+          Band.all.tally('managers.n')
+
 
 Eager Loading
 =============

source/release-notes/mongoid-8.0.txt

@@ -527,6 +527,7 @@ it was stored and returned as a String.
   # => Mongoid 7: "data!"
   # => Mongoid 8: <BSON::Binary:0x2600 type=generic data=0x6461746121...>
 
+
 ``Decimal128``-backed ``BigDecimal`` fields
 -------------------------------------------
 
@@ -540,3 +541,35 @@ feature flag turned off, values assigned to fields of type ``BigDecimal`` are
 stored as Strings. See the section on :ref:`BigDecimal Fields <bigdecimal-fields>`
 for more details.
 
+
+Implemented ``.tally`` method on ``Mongoid#Criteria``
+-----------------------------------------------------
+
+Mongoid 8 implements the ``.tally`` method on ``Mongoid#Criteria``. ``tally``
+takes a field name as a parameter and returns a mapping from values to their
+counts. For example, take the following model:
+
+.. code::
+
+  class User
+    include Mongoid::Document
+    field :age
+  end
+
+and the following documents in the database:
+
+.. code::
+
+  { _id: 1, age: 21 }
+  { _id: 2, age: 21 }
+  { _id: 3, age: 22 }
+
+Calling ``tally`` on the age field yields the following:
+
+.. code::
+
+  User.tally("age")
+  # => { 21 => 2, 22 => 1 }
+
+The ``tally`` method accepts the dot notation and field aliases. It also
+allows for tallying localized fields.