Deprecate table names containing dots

Dots have special meaning in most backends (e.g. everything except
SQLite3), as well as most methods that work with table or column names.
This isn't something that we ever explicitly supported, but there's at
least one case of somebody using this (see #24367), so we'll go through a deprecation
cycle as normal.

activerecord/lib/active_record/model_schema.rb

@@ -144,6 +144,13 @@ module ActiveRecord
       def table_name=(value)
         value = value && value.to_s
 
+        if value && value.include?(".")
+          # When this deprecation warning is removed, revert commit 04ac5655be91f49cd4dfe2838df96213502fb274
+          ActiveSupport::Deprecation.warn(
+            'Support for table names containing "." is deprecated and will be removed in Rails 5.1.'
+          )
+        end
+
         if defined?(@table_name)
           return if value == @table_name
           reset_column_information if connected?

activerecord/test/cases/base_test.rb

@@ -1531,4 +1531,12 @@ class BasicsTest < ActiveRecord::TestCase
     assert Developer.new.respond_to?(:last_name=)
     assert Developer.new.respond_to?(:last_name?)
   end
+
+  test "table names containing a dot are deprecated" do
+    assert_deprecated do
+      Class.new(ActiveRecord::Base) do
+        self.table_name = "foo.bar"
+      end
+    end
+  end
 end