Revert the behavior of booleans in string columns to that of 4.1

Why are people assigning booleans to string columns? >_>

We unintentionally changed the behavior on Sqlite3 and PostgreSQL.
Boolean values should cast to the database's representation of true and
false. This is 't' and 'f' by default, and "1" and "0" on Mysql. The
implementation to make the connection adapter specific behavior is hacky
at best, and should be re-visted once we decide how we actually want to
separate the concerns related to things that should change based on the
database adapter.

That said, this isn't something I'd expect to change based on my
database adapter. We're storing a string, so the way the database
represents a boolean should be irrelevant. It also seems strange for us
to give booleans special behavior at all in string columns. Why is
`to_s` not sufficient? It's inconsistent and confusing. Perhaps we
should consider deprecating in the future.

Fixes #17571

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -648,6 +648,8 @@ def valid_type?(type)
       def initialize_type_map(m) # :nodoc:
         super
 
+        register_class_with_limit m, %r(char)i, MysqlString
+
         m.register_type %r(tinytext)i,   Type::Text.new(limit: 2**8 - 1)
         m.register_type %r(tinyblob)i,   Type::Binary.new(limit: 2**8 - 1)
         m.register_type %r(text)i,       Type::Text.new(limit: 2**16 - 1)
@@ -672,7 +674,7 @@ def initialize_type_map(m) # :nodoc:
         m.register_type(%r(enum)i) do |sql_type|
           limit = sql_type[/^enum\((.+)\)/i, 1]
             .split(',').map{|enum| enum.strip.length - 2}.max
-          Type::String.new(limit: limit)
+          MysqlString.new(limit: limit)
         end
       end
 
@@ -847,6 +849,26 @@ def extract_foreign_key_action(structure, name, action) # :nodoc:
           end
         end
       end
+
+      class MysqlString < Type::String # :nodoc:
+        def type_cast_for_database(value)
+          case value
+          when true then "1"
+          when false then "0"
+          else super
+          end
+        end
+
+        private
+
+        def cast_value(value)
+          case value
+          when true then "1"
+          when false then "0"
+          else super
+          end
+        end
+      end
     end
   end
 end

activerecord/lib/active_record/type/string.rb

@@ -15,8 +15,8 @@ def type_cast_for_database(value)
         case value
         when ::Numeric, ActiveSupport::Duration then value.to_s
         when ::String then ::String.new(value)
-        when true then "1"
-        when false then "0"
+        when true then "t"
+        when false then "f"
         else super
         end
       end
@@ -25,8 +25,8 @@ def type_cast_for_database(value)
 
       def cast_value(value)
         case value
-        when true then "1"
-        when false then "0"
+        when true then "t"
+        when false then "f"
         # String.new is slightly faster than dup
         else ::String.new(value.to_s)
         end

activerecord/test/cases/type/string_test.rb

@@ -4,8 +4,8 @@ module ActiveRecord
   class StringTypeTest < ActiveRecord::TestCase
     test "type casting" do
       type = Type::String.new
-      assert_equal "1", type.type_cast_from_user(true)
-      assert_equal "0", type.type_cast_from_user(false)
+      assert_equal "t", type.type_cast_from_user(true)
+      assert_equal "f", type.type_cast_from_user(false)
       assert_equal "123", type.type_cast_from_user(123)
     end