Apply monkey patching only after loading a MySQL adapter.

config/initializers/ar_mysql_column_charset.rb

@@ -1,3 +1 @@
-ActiveSupport.on_load :active_record do
-  require 'ar_mysql_column_charset'
-end
+require 'ar_mysql_column_charset'

lib/ar_mysql_column_charset.rb

@@ -1,121 +1,16 @@
-require 'active_record'
-
 # Module#prepend support for Ruby 1.9
 require 'prepend' unless Module.method_defined?(:prepend)
 
-module ActiveRecord::ConnectionAdapters
-  class ColumnDefinition
-    module CharsetSupport
-      attr_accessor :charset, :collation
-    end
-
-    prepend CharsetSupport
-  end
-
-  class TableDefinition
-    module CharsetSupport
-      def new_column_definition(name, type, options)
-        column = super
-        column.charset   = options[:charset]
-        column.collation = options[:collation]
-        column
-      end
-    end
-
-    prepend CharsetSupport
-  end
-
-  class AbstractMysqlAdapter
-    module CharsetSupport
-      def prepare_column_options(column, types)
-        spec = super
-        spec[:charset]   = column.charset.inspect if column.charset && column.charset != charset
-        spec[:collation] = column.collation.inspect if column.collation && column.collation != collation
-        spec
-      end
+require 'active_support'
 
-      def migration_keys
-        super + [:charset, :collation]
-      end
-
-      def utf8mb4_supported?
-        if @utf8mb4_supported.nil?
-          @utf8mb4_supported = !select("show character set like 'utf8mb4'").empty?
-        else
-          @utf8mb4_supported
+ActiveSupport.on_load :active_record do
+  class << ActiveRecord::Base
+    def establish_connection(spec = nil)
+      super.tap { |ret|
+        if /mysql/i === connection.adapter_name
+          require 'ar_mysql_column_charset/main'
         end
-      end
-
-      def charset_collation(charset, collation)
-        [charset, collation].map { |name|
-          case name
-          when nil
-            nil
-          when /\A(utf8mb4(_\w*)?)\z/
-            if utf8mb4_supported?
-              $1
-            else
-              "utf8#{$2}"
-            end
-          else
-            name.to_s
-          end
-        }
-      end
-
-      def create_database(name, options = {})
-        # utf8mb4 is used in column definitions; use utf8 for
-        # databases.
-        [:charset, :collation].each { |key|
-          case options[key]
-          when /\A(utf8mb4(_\w*)?)\z/
-            options = options.merge(key => "utf8#{$2}")
-          end
-        }
-        super(name, options)
-      end
-    end
-
-    prepend CharsetSupport
-
-    class SchemaCreation
-      module CharsetSupport
-        def column_options(o)
-          column_options = super
-          column_options[:charset]   = o.charset unless o.charset.nil?
-          column_options[:collation] = o.collation unless o.collation.nil?
-          column_options
-        end
-
-        def add_column_options!(sql, options)
-          charset, collation = @conn.charset_collation(options[:charset], options[:collation])
-
-          if charset
-            sql << " CHARACTER SET #{charset}"
-          end
-
-          if collation
-            sql << " COLLATE #{collation}"
-          end
-
-          super
-        end
-      end
-
-      prepend CharsetSupport
-    end
-
-    class Column
-      module CharsetSupport
-        attr_reader :charset
-
-        def initialize(*args)
-          super
-          @charset = @collation[/\A[^_]+/] unless @collation.nil?
-        end
-      end
-
-      prepend CharsetSupport
+      }
     end
   end
 end

lib/ar_mysql_column_charset/main.rb

@@ -0,0 +1,118 @@
+raise "Do not directly load this library." unless defined?(ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter)
+
+module ActiveRecord::ConnectionAdapters
+  class ColumnDefinition
+    module CharsetSupport
+      attr_accessor :charset, :collation
+    end
+
+    prepend CharsetSupport
+  end
+
+  class TableDefinition
+    module CharsetSupport
+      def new_column_definition(name, type, options)
+        column = super
+        column.charset   = options[:charset]
+        column.collation = options[:collation]
+        column
+      end
+    end
+
+    prepend CharsetSupport
+  end
+
+  class AbstractMysqlAdapter
+    module CharsetSupport
+      def prepare_column_options(column, types)
+        spec = super
+        spec[:charset]   = column.charset.inspect if column.charset && column.charset != charset
+        spec[:collation] = column.collation.inspect if column.collation && column.collation != collation
+        spec
+      end
+
+      def migration_keys
+        super + [:charset, :collation]
+      end
+
+      def utf8mb4_supported?
+        if @utf8mb4_supported.nil?
+          @utf8mb4_supported = !select("show character set like 'utf8mb4'").empty?
+        else
+          @utf8mb4_supported
+        end
+      end
+
+      def charset_collation(charset, collation)
+        [charset, collation].map { |name|
+          case name
+          when nil
+            nil
+          when /\A(utf8mb4(_\w*)?)\z/
+            if utf8mb4_supported?
+              $1
+            else
+              "utf8#{$2}"
+            end
+          else
+            name.to_s
+          end
+        }
+      end
+
+      def create_database(name, options = {})
+        # utf8mb4 is used in column definitions; use utf8 for
+        # databases.
+        [:charset, :collation].each { |key|
+          case options[key]
+          when /\A(utf8mb4(_\w*)?)\z/
+            options = options.merge(key => "utf8#{$2}")
+          end
+        }
+        super(name, options)
+      end
+    end
+
+    prepend CharsetSupport
+
+    class SchemaCreation
+      module CharsetSupport
+        def column_options(o)
+          column_options = super
+          column_options[:charset]   = o.charset unless o.charset.nil?
+          column_options[:collation] = o.collation unless o.collation.nil?
+          column_options
+        end
+
+        def add_column_options!(sql, options)
+          charset, collation = @conn.charset_collation(options[:charset], options[:collation])
+
+          if charset
+            sql << " CHARACTER SET #{charset}"
+          end
+
+          if collation
+            sql << " COLLATE #{collation}"
+          end
+
+          super
+        end
+      end
+
+      prepend CharsetSupport
+    end
+
+    class Column
+      module CharsetSupport
+        attr_reader :charset
+
+        def initialize(*args)
+          super
+          @charset = @collation[/\A[^_]+/] unless @collation.nil?
+        end
+      end
+
+      prepend CharsetSupport
+    end
+  end
+end