Compute table name based on model name

Instead of using the Active Record Base's class name, the table name is
now inferred based on its model name object. This allows customization
of the table name pattern since a distinct instance of
`ActiveModel::Name` can be used instead.

For example, to map a `PostRecord` class to a `posts` table:

```ruby

 class PostRecord < ActiveRecord::Base
   class << self
     def model_name
       ActiveModel::Name.new(self, nil, 'Post')
     end
   end
 end

 PostRecord.table_name
 # => "posts"
```

activerecord/lib/active_record/model_schema.rb

@@ -213,6 +213,21 @@ module ActiveRecord
       # the table name guess for an Invoice class becomes "myapp_invoices".
       # Invoice::Lineitem becomes "myapp_invoice_lineitems".
       #
+      # Active Model Naming's +model_name+ is the base name used to guess the
+      # table name. In case a custom Active Model Name is defined, it will be
+      # used for the table name as well:
+      #
+      #   class PostRecord < ActiveRecord::Base
+      #     class << self
+      #       def model_name
+      #         ActiveModel::Name.new(self, nil, "Post")
+      #       end
+      #     end
+      #   end
+      #
+      #   PostRecord.table_name
+      #   # => "posts"
+      #
       # You can also set your own table name explicitly:
       #
       #   class Mouse < ActiveRecord::Base
@@ -587,8 +602,8 @@ module ActiveRecord
         end
 
         # Guesses the table name, but does not decorate it with prefix and suffix information.
-        def undecorated_table_name(class_name = base_class.name)
-          table_name = class_name.to_s.demodulize.underscore
+        def undecorated_table_name(model_name)
+          table_name = model_name.to_s.demodulize.underscore
           pluralize_table_names ? table_name.pluralize : table_name
         end
 
@@ -602,7 +617,7 @@ module ActiveRecord
               contained += "_"
             end
 
-            "#{full_table_name_prefix}#{contained}#{undecorated_table_name(name)}#{full_table_name_suffix}"
+            "#{full_table_name_prefix}#{contained}#{undecorated_table_name(model_name)}#{full_table_name_suffix}"
           else
             # STI subclasses always use their superclass' table.
             base_class.table_name

activerecord/test/cases/attribute_methods_test.rb

@@ -965,6 +965,7 @@ class AttributeMethodsTest < ActiveRecord::TestCase
 
   test "define_attribute_method works with both symbol and string" do
     klass = Class.new(ActiveRecord::Base)
+    klass.table_name = "foo"
 
     assert_nothing_raised { klass.define_attribute_method(:foo) }
     assert_nothing_raised { klass.define_attribute_method("bar") }

activerecord/test/cases/base_test.rb

@@ -505,6 +505,10 @@ class BasicsTest < ActiveRecord::TestCase
     Post.reset_table_name
   end
 
+  def test_table_name_based_on_model_name
+    assert_equal "posts", PostRecord.table_name
+  end
+
   def test_null_fields
     assert_nil Topic.find(1).parent_id
     assert_nil Topic.create("title" => "Hey you").parent_id
@@ -1478,6 +1482,7 @@ class BasicsTest < ActiveRecord::TestCase
 
   test "scoped can take a values hash" do
     klass = Class.new(ActiveRecord::Base)
+    klass.table_name = "bar"
     assert_equal ["foo"], klass.all.merge!(select: "foo").select_values
   end
 

activerecord/test/cases/primary_keys_test.rb

@@ -174,6 +174,7 @@ class PrimaryKeysTest < ActiveRecord::TestCase
 
   def test_quoted_primary_key_after_set_primary_key
     k = Class.new(ActiveRecord::Base)
+    k.table_name = "bar"
     assert_equal k.connection.quote_column_name("id"), k.quoted_primary_key
     k.primary_key = "foo"
     assert_equal k.connection.quote_column_name("foo"), k.quoted_primary_key

activerecord/test/models/post.rb

@@ -382,3 +382,11 @@ class Postesque < ActiveRecord::Base
   belongs_to :author_with_address, class_name: "Author", foreign_key: :author_id
   belongs_to :author_with_the_letter_a, class_name: "Author", foreign_key: :author_id
 end
+
+class PostRecord < ActiveRecord::Base
+  class << self
+    def model_name
+      ActiveModel::Name.new(self, nil, "Post")
+    end
+  end
+end

guides/source/active_support_core_extensions.md

@@ -1461,8 +1461,8 @@ Active Record uses this method to compute the default table name that correspond
 
 ```ruby
 # active_record/model_schema.rb
-def undecorated_table_name(class_name = base_class.name)
-  table_name = class_name.to_s.demodulize.underscore
+def undecorated_table_name(model_name)
+  table_name = model_name.to_s.demodulize.underscore
   pluralize_table_names ? table_name.pluralize : table_name
 end
 ```