Fix duplicate escaping of quotes for check constraint expressions in …

…MySQL schema

Mysql automatically escapes quotes in generated check constraints expressions.
When Rails dumps the schema (into schema.rb) the generated schema also contains,
the quotes with duplicated escaping (\\\\').
To fix this, we remove the escaping, that MySQL provides from the fetched
expression.

activerecord/CHANGELOG.md

@@ -1,3 +1,12 @@
+*   Fix duplicate quoting for check constraint expressions in schema dump when using MySQL
+
+    A check constraint with an expression, that already contains quotes, lead to an invalid schema
+    dump with the mysql2 adapter.
+
+    Fixes #42424.
+
+    *Felix Tscheulin*
+
 *   Performance tune the SQLite3 adapter connection configuration
 
     For Rails applications, the Write-Ahead-Log in normal syncing mode with a capped journal size, a healthy shared memory buffer and a shared cache will perform, on average, 2× better.

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -531,6 +531,13 @@ def check_constraints(table_name)
             expression = row["expression"]
             expression = expression[1..-2] if expression.start_with?("(") && expression.end_with?(")")
             expression = strip_whitespace_characters(expression)
+
+            unless mariadb?
+              # MySQL returns check constraints expression in an already escaped form.
+              # This leads to duplicate escaping later (e.g. when the expression is used in the SchemaDumper).
+              expression = expression.gsub("\\'", "'")
+            end
+
             CheckConstraintDefinition.new(table_name, expression, options)
           end
         else

activerecord/test/cases/adapters/mysql2/check_constraint_quoting_test.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "support/schema_dumping_helper"
+
+if ActiveRecord::Base.connection.supports_check_constraints?
+  class Mysql2CheckConstraintQuotingTest < ActiveRecord::Mysql2TestCase
+    include SchemaDumpingHelper
+
+    setup do
+      @connection = ActiveRecord::Base.connection
+      @connection.create_table "trades", force: true do |t|
+        t.string :name
+      end
+    end
+
+    teardown do
+      @connection.drop_table "trades", if_exists: true rescue nil
+    end
+
+    def test_check_constraint_no_duplicate_expression_quoting
+      @connection.add_check_constraint :trades, "name != 'forbidden_string'"
+
+      check_constraints = @connection.check_constraints("trades")
+      assert_equal 1, check_constraints.size
+
+      expression = check_constraints.first.expression
+      if ActiveRecord::Base.connection.mariadb?
+        assert_equal "`name` <> 'forbidden_string'", expression
+      else
+        assert_equal "`name` <> _utf8mb4'forbidden_string'", expression
+      end
+    end
+  end
+end