Add support for if_exists option when removing a check constraint

The `remove_check_constraint` method now accepts an `if_exists` option. If set
to true an error won't be raised if the check constraint doesn't exist.

This commit is a combination of PR rails#45726 and rails#45718 with some
additional changes to improve wording, testing, and implementation.

Usage:

```ruby
remove_check_constraint :products, name: "price_check", if_exists: true
```

Fixes rails#45634

Co-authored-by: Margaret Parsa <[email protected]>
Co-authored-by: Aditya Bhutani <[email protected]>

Author: 3 people

activerecord/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Adds support for `if_exists` option when removing a check constraint.
+
+    The `remove_check_constraint` method now accepts an `if_exists` option. If set
+    to true an error won't be raised if the check constraint doesn't exist.
+
+    *Margaret Parsa* and *Aditya Bhutani*
+
 *   `find_or_create_by` now try to find a second time if it hits a unicity constraint.
 
     `find_or_create_by` always has been inherently racy, either creating multiple

activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb

@@ -889,6 +889,17 @@ def remove_check_constraint(*args, **options)
         @base.remove_check_constraint(name, *args, **options)
       end
 
+      # Checks if a check_constraint exists on a table.
+      #
+      #  unless t.check_constraint_exists?(name: "price_check")
+      #    t.check_constraint("price > 0", name: "price_check")
+      #  end
+      #
+      # See {connection.check_constraint_exists?}[rdoc-ref:SchemaStatements#check_constraint_exists?]
+      def check_constraint_exists?(*args, **options)
+        @base.check_constraint_exists?(name, *args, **options)
+      end
+
       private
         def raise_on_if_exist_options(options)
           unrecognized_option = options.keys.find do |key|

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -1227,16 +1227,24 @@ def check_constraint_options(table_name, expression, options) # :nodoc:
         options
       end
 
-      # Removes the given check constraint from the table.
+      # Removes the given check constraint from the table. Removing a check constraint
+      # that does not exist will raise an error.
       #
       #   remove_check_constraint :products, name: "price_check"
       #
+      # To silently ignore a non-existent check constraint rather than raise an error,
+      # use the `if_exists` option.
+      #
+      #   remove_check_constraint :products, name: "price_check", if_exists: true
+      #
       # The +expression+ parameter will be ignored if present. It can be helpful
       # to provide this in a migration's +change+ method so it can be reverted.
       # In that case, +expression+ will be used by #add_check_constraint.
       def remove_check_constraint(table_name, expression = nil, **options)
         return unless supports_check_constraints?
 
+        return if options[:if_exists] && !check_constraint_exists?(table_name, **options)
+
         chk_name_to_delete = check_constraint_for!(table_name, expression: expression, **options).name
 
         at = create_alter_table(table_name)
@@ -1437,6 +1445,10 @@ def use_foreign_keys?
       end
 
       private
+	def check_constraint_exists?(table_name, **options)
+	  check_constraint_for(table_name, **options).present?
+	end
+
         def validate_change_column_null_argument!(value)
           unless value == true || value == false
             raise ArgumentError, "change_column_null expects a boolean value (true for NULL, false for NOT NULL). Got: #{value.inspect}"

activerecord/lib/active_record/connection_adapters/sqlite3/schema_statements.rb

@@ -103,6 +103,8 @@ def add_check_constraint(table_name, expression, **options)
         end
 
         def remove_check_constraint(table_name, expression = nil, **options)
+          return if options[:if_exists] && !check_constraint_exists?(table_name, **options)
+
           check_constraints = check_constraints(table_name)
           chk_name_to_delete = check_constraint_for!(table_name, expression: expression, **options).name
           check_constraints.delete_if { |chk| chk.name == chk_name_to_delete }

activerecord/test/cases/migration/check_constraint_test.rb

@@ -184,6 +184,22 @@ def test_remove_check_constraint
           assert_empty @connection.check_constraints("trades")
         end
 
+        def test_removing_check_constraint_with_if_exists_option
+          @connection.add_check_constraint :trades, "quantity > 0", name: "quantity_check"
+
+          @connection.remove_check_constraint :trades, name: "quantity_check"
+
+          error = assert_raises ArgumentError do
+            @connection.remove_check_constraint :trades, name: "quantity_check"
+          end
+
+          assert_equal "Table 'trades' has no check constraint for {:name=>\"quantity_check\"}", error.message
+
+          assert_nothing_raised do
+            @connection.remove_check_constraint :trades, name: "quantity_check", if_exists: true
+          end
+        end
+
         def test_remove_non_existing_check_constraint
           assert_raises(ArgumentError) do
             @connection.remove_check_constraint :trades, name: "nonexistent"