add missing foreign key actions, fixes #76

Author: tanner0101

Sources/PostgreSQL/Query/PostgreSQLQuery+ForeignKeyAction.swift

@@ -1,13 +1,39 @@
 extension PostgreSQLQuery {
+    /// In addition, when the data in the referenced columns is changed, certain actions are performed on the data in this table's
+    /// columns. The ON DELETE clause specifies the action to perform when a referenced row in the referenced table is being deleted.
+    /// Likewise, the ON UPDATE clause specifies the action to perform when a referenced column in the referenced table is being updated
+    /// to a new value. If the row is updated, but the referenced column is not actually changed, no action is done. Referential actions
+    /// other than the NO ACTION check cannot be deferred, even if the constraint is declared deferrable. There are the following possible
+    /// actions for each clause.
+    ///
+    /// https://www.postgresql.org/docs/10/static/sql-createtable.html.
     public enum ForeignKeyAction {
-        case nullify
+        /// Produce an error indicating that the deletion or update would create a foreign key constraint violation.
+        /// If the constraint is deferred, this error will be produced at constraint check time if there still exist
+        /// any referencing rows. This is the default action.
+        case noAction
+        /// Produce an error indicating that the deletion or update would create a foreign key constraint violation.
+        /// This is the same as NO ACTION except that the check is not deferrable.
+        case restrict
+        /// Delete any rows referencing the deleted row, or update the values of the referencing column(s) to the new
+        // values of the referenced columns, respectively.
+        case cascade
+        /// Set the referencing column(s) to null.
+        case setNull
+        /// Set the referencing column(s) to their default values. (There must be a row in the referenced table matching
+        /// the default values, if they are not null, or the operation will fail.)
+        case setDefault
     }
 }
 
 extension PostgreSQLSerializer {
     internal func serialize(_ action: PostgreSQLQuery.ForeignKeyAction) -> String {
         switch action {
-        case .nullify: return "NULLIFY"
+        case .noAction: return "NO ACTION"
+        case .restrict: return "RESTRICT"
+        case .cascade: return "CASCADE"
+        case .setNull: return "SET NULL"
+        case .setDefault: return "SET DEFAULT"
         }
     }
 }