1 files changed, 35 insertions, 0 deletions
diff --git a/spec/support/helpers/migrations_helpers/schema_version_finder.rb b/spec/support/helpers/migrations_helpers/schema_version_finder.rb
new file mode 100644
index 00000000000..69469959ce5
--- /dev/null
+++ b/spec/support/helpers/migrations_helpers/schema_version_finder.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+# Sometimes data migration specs require adding invalid test data in order to test
+# the migration (e.g. adding a row with null foreign key). Certain db migrations that
+# add constraints (e.g. NOT NULL constraint) prevent invalid records from being added
+# and data migration from being tested. For this reason, SchemaVersionFinder can be used
+# to find and use schema prior to specified one.
+#
+# @example
+#   RSpec.describe CleanupThings, :migration,
+#     schema: MigrationHelpers::SchemaVersionFinder.migration_prior(AddNotNullConstraint) do ...
+#
+# SchemaVersionFinder returns schema version prior to the one specified, which allows to then add
+# invalid records to the database, which in return allows to properly test data migration.
+module MigrationHelpers
+  class SchemaVersionFinder
+    def self.migrations_paths
+      ActiveRecord::Migrator.migrations_paths
+    end
+
+    def self.migration_context
+      ActiveRecord::MigrationContext.new(migrations_paths, ActiveRecord::SchemaMigration)
+    end
+
+    def self.migrations
+      migration_context.migrations
+    end
+
+    def self.migration_prior(migration_klass)
+      migrations.each_cons(2) do |previous, migration|
+        break previous.version if migration.name == migration_klass.name
+      end
+    end
+  end
+end