Add support for `if_exists/if_not_exists` on `remove_foreign_key/add_foreign_key`

Applications can set their migrations to ignore exceptions raised when adding a foreign key that already exists or when removing a foreign key that does not exist.

Add test cases

💇‍♀️

activerecord/CHANGELOG.md

@@ -1,3 +1,28 @@
+*   Adds support for `if_not_exists` to `add_foreign_key` and `if_exists` to `remove_foreign_key`.
+
+    Applications can set their migrations to ignore exceptions raised when adding a foreign key
+    that already exists or when removing a foreign key that does not exist.
+
+    Example Usage:
+
+    ```ruby
+    class AddAuthorsForeignKeyToArticles < ActiveRecord::Migration[7.0]
+      def change
+        add_foreign_key :articles, :authors, if_not_exists: true
+      end
+    end
+    ```
+
+    ```ruby
+    class RemoveAuthorsForeignKeyFromArticles < ActiveRecord::Migration[7.0]
+      def change
+        remove_foreign_key :articles, :authors, if_exists: true
+      end
+    end
+    ```
+
+    *Roberto Miranda*
+
 *   Prevent polluting ENV during postgresql structure dump/load
 
     Some configuration parameters were provided to pg_dump / psql via

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

@@ -1039,6 +1039,10 @@ module ActiveRecord
       #
       #   ALTER TABLE "articles" ADD CONSTRAINT fk_rails_e74ce85cbc FOREIGN KEY ("author_id") REFERENCES "authors" ("id")
       #
+      # ====== Creating a foreign key, ignoring method call if the foreign key exists
+      #
+      #  add_foreign_key(:articles, :authors, if_not_exists: true)
+      #
       # ====== Creating a foreign key on a specific column
       #
       #   add_foreign_key :articles, :users, column: :author_id, primary_key: "lng_id"
@@ -1066,10 +1070,14 @@ module ActiveRecord
       #   Action that happens <tt>ON DELETE</tt>. Valid values are +:nullify+, +:cascade+ and +:restrict+
       # [<tt>:on_update</tt>]
       #   Action that happens <tt>ON UPDATE</tt>. Valid values are +:nullify+, +:cascade+ and +:restrict+
+      # [<tt>:if_not_exists</tt>]
+      #   Specifies if the foreign key already exists to not try to re-add it. This will avoid
+      #   duplicate column errors.
       # [<tt>:validate</tt>]
       #   (PostgreSQL only) Specify whether or not the constraint should be validated. Defaults to +true+.
       def add_foreign_key(from_table, to_table, **options)
         return unless supports_foreign_keys?
+        return if options[:if_not_exists] == true && foreign_key_exists?(from_table, to_table)
 
         options = foreign_key_options(from_table, to_table, options)
         at = create_alter_table from_table
@@ -1099,12 +1107,18 @@ module ActiveRecord
       #
       #   remove_foreign_key :accounts, name: :special_fk_name
       #
+      # Checks if the foreign key exists before trying to remove it. Will silently ignore indexes that
+      # don't exist.
+      #
+      #   remove_foreign_key :accounts, :branches, if_exists: true
+      #
       # The +options+ hash accepts the same keys as SchemaStatements#add_foreign_key
       # with an addition of
       # [<tt>:to_table</tt>]
       #   The name of the table that contains the referenced primary key.
       def remove_foreign_key(from_table, to_table = nil, **options)
         return unless supports_foreign_keys?
+        return if options[:if_exists] == true && !foreign_key_exists?(from_table, to_table)
 
         fk_name_to_delete = foreign_key_for!(from_table, to_table: to_table, **options).name
 

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

@@ -60,6 +60,8 @@ module ActiveRecord
         end
 
         def remove_foreign_key(from_table, to_table = nil, **options)
+          return if options[:if_exists] == true && !foreign_key_exists?(from_table, to_table)
+
           to_table ||= options[:to_table]
           options = options.except(:name, :to_table, :validate)
           foreign_keys = foreign_keys(from_table)

activerecord/test/cases/migration/foreign_key_test.rb

@@ -575,6 +575,67 @@ if ActiveRecord::Base.connection.supports_foreign_keys?
           silence_stream($stdout) { migration.migrate(:down) }
           ActiveRecord::Base.table_name_suffix = nil
         end
+
+        def test_remove_foreign_key_with_if_exists_not_set
+          @connection.add_foreign_key :astronauts, :rockets
+          assert_equal 1, @connection.foreign_keys("astronauts").size
+
+          @connection.remove_foreign_key :astronauts, :rockets
+          assert_equal [], @connection.foreign_keys("astronauts")
+
+          error = assert_raises do
+            @connection.remove_foreign_key :astronauts, :rockets
+          end
+
+          assert_equal("Table 'astronauts' has no foreign key for rockets", error.message)
+        end
+
+        def test_remove_foreign_key_with_if_exists_set
+          @connection.add_foreign_key :astronauts, :rockets
+          assert_equal 1, @connection.foreign_keys("astronauts").size
+
+          @connection.remove_foreign_key :astronauts, :rockets
+          assert_equal [], @connection.foreign_keys("astronauts")
+
+          assert_nothing_raised do
+            @connection.remove_foreign_key :astronauts, :rockets, if_exists: true
+          end
+        end
+
+        def test_add_foreign_key_with_if_not_exists_not_set
+          @connection.add_foreign_key :astronauts, :rockets
+          assert_equal 1, @connection.foreign_keys("astronauts").size
+
+          if current_adapter?(:SQLite3Adapter)
+            assert_nothing_raised do
+              @connection.add_foreign_key :astronauts, :rockets
+            end
+          else
+            error = assert_raises do
+              @connection.add_foreign_key :astronauts, :rockets
+            end
+
+            if current_adapter?(:Mysql2Adapter)
+              if ActiveRecord::Base.connection.mariadb?
+                assert_match(/Duplicate key on write or update/, error.message)
+              else
+                assert_match(/Duplicate foreign key constraint name/, error.message)
+              end
+            else
+              assert_match(/PG::DuplicateObject: ERROR:.*for relation "astronauts" already exists/, error.message)
+            end
+
+          end
+        end
+
+        def test_add_foreign_key_with_if_not_exists_set
+          @connection.add_foreign_key :astronauts, :rockets
+          assert_equal 1, @connection.foreign_keys("astronauts").size
+
+          assert_nothing_raised do
+            @connection.add_foreign_key :astronauts, :rockets, if_not_exists: true
+          end
+        end
       end
     end
   end