Expose foreign key name ignore pattern in configuration

activerecord/lib/active_record.rb

@@ -42,6 +42,7 @@ module ActiveRecord
   autoload :CounterCache
   autoload :DynamicMatchers
   autoload :Enum
+  autoload :ForeignKeys
   autoload :InternalMetadata
   autoload :Explain
   autoload :Inheritance

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

@@ -101,6 +101,10 @@ def validate?
       end
       alias validated? validate?
 
+      def export_name_on_schema_dump?
+        name !~ ActiveRecord::SchemaDumper.fk_ignore_pattern
+      end
+
       def defined_for?(to_table_ord = nil, to_table: nil, **options)
         if to_table_ord
           self.to_table == to_table_ord.to_s

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

@@ -1324,7 +1324,7 @@ def foreign_key_name(table_name, options)
             identifier = "#{table_name}_#{options.fetch(:column)}_fk"
             hashed_identifier = Digest::SHA256.hexdigest(identifier).first(10)
 
-            "fk_rails_#{hashed_identifier}"
+            "#{ActiveRecord::ForeignKeys::PREFIX}_#{hashed_identifier}"
           end
         end
 

activerecord/lib/active_record/foreign_keys.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+module ActiveRecord
+  module ForeignKeys
+    # The prefix used by Rails to name unnamed foreign keys.
+    PREFIX = "fk_rails"
+
+    # Default regular expression used by Rails to determine if a foreign key
+    # name was generated.
+    DEFAULT_IGNORE_PATTERN = /^#{PREFIX}_[0-9a-f]{10}$/
+  end
+end

activerecord/lib/active_record/schema_dumper.rb

@@ -17,6 +17,12 @@ class SchemaDumper #:nodoc:
     # Only strings are accepted if ActiveRecord::Base.schema_format == :sql.
     cattr_accessor :ignore_tables, default: []
 
+    ##
+    # :singleton-method:
+    # Specify a custom regular expression matching foreign keys which name
+    # should not be dumped to db/schema.rb.
+    cattr_accessor :fk_ignore_pattern, default: ActiveRecord::ForeignKeys::DEFAULT_IGNORE_PATTERN
+
     class << self
       def dump(connection = ActiveRecord::Base.connection, stream = STDOUT, config = ActiveRecord::Base)
         connection.create_schema_dumper(generate_options(config)).dump(stream)
@@ -210,7 +216,7 @@ def foreign_keys(table, stream)
               parts << "primary_key: #{foreign_key.primary_key.inspect}"
             end
 
-            if foreign_key.name !~ /^fk_rails_[0-9a-f]{10}$/
+            if foreign_key.export_name_on_schema_dump?
               parts << "name: #{foreign_key.name.inspect}"
             end
 

activerecord/test/cases/migration/foreign_key_test.rb

@@ -306,6 +306,17 @@ def test_schema_dumping_with_options
           assert_match %r{\s+add_foreign_key "fk_test_has_fk", "fk_test_has_pk", column: "fk_id", primary_key: "pk_id", name: "fk_name"$}, output
         end
 
+        def test_schema_dumping_with_custom_fk_ignore_pattern
+          original_pattern = ActiveRecord::SchemaDumper.fk_ignore_pattern
+          ActiveRecord::SchemaDumper.fk_ignore_pattern = /^ignored_/
+          @connection.add_foreign_key :astronauts, :rockets, name: :ignored_fk_astronauts_rockets
+
+          output = dump_table_schema "astronauts"
+          assert_match %r{\s+add_foreign_key "astronauts", "rockets"$}, output
+
+          ActiveRecord::SchemaDumper.fk_ignore_pattern = original_pattern
+        end
+
         def test_schema_dumping_on_delete_and_on_update_options
           @connection.add_foreign_key :astronauts, :rockets, column: "rocket_id", on_delete: :nullify, on_update: :cascade
 

guides/source/configuring.md

@@ -400,10 +400,16 @@ by adding the following to your `application.rb` file:
     Rails.application.config.active_record.sqlite3.represent_boolean_as_integer = true
     ```
 
-The schema dumper adds one additional configuration option:
+The schema dumper adds two additional configuration options:
 
 * `ActiveRecord::SchemaDumper.ignore_tables` accepts an array of tables that should _not_ be included in any generated schema file.
 
+* `ActiveRecord::SchemaDumper.fk_ignore_pattern` allows setting a different regular
+  expression that will be used to decide whether a foreign key's name should be
+  dumped to db/schema.rb or not. By default, foreign key names starting with
+  `fk_rails_` are not exported to the database schema dump.
+  Defaults to `/^fk_rails_[0-9a-f]{10}$/`.
+
 ### Configuring Action Controller
 
 `config.action_controller` includes a number of configuration settings: