Interceptor Signals

Phrag can signal configured functions per table & operation type at pre/post DB operation time. This is where things like access controls or custom business logics can be configured. Signal functions are called with different parameters as below:

Pre-operation Interceptor Function

Type	Signal function receives (as first parameter):	Returned value will be:
query	Lacinia's selection map including fields such as :arguments and :selections.	Passed to subsequent query operation.
create	Submitted mutation parameters	Passed to subsequent create operation.
update	Submitted mutation parameters	Passed to subsequent update operation.
delete	Submitted mutation parameters	Passed to subsequent delete operation.

Notes:

• query signal functions for matching table will be called in nested queries (relations) as well.

Post-operation Interceptor Function

Type	Signal function receives (as a first parameter):	Returned value will be:
query	Result values returned from query operation.	Passed to response body.
create	Primary key object of created item: e.g. {:id 3}	Passed to response body.
update	Result object: {:result true}	Passed to response body.
delete	Result object: {:result true}	Passed to response body.

Signal Context Map

All receiver functions will have a context map as its second argument. It'd contain a signal context specified in a Phrag config (:signal-ctx) together with a DB connection (:db) and an incoming HTTP request (:req).

Examples

(defn- end-user-access
  "Users can query only his/her own user info"
  [selection ctx]
  (let [user (user-info (:req ctx))]
    (if (admin-user? user))
      selection
      (update-in selection [:arguments :where :user_id] {:eq (:id user)})))

(defn- hide-internal-id
  "Removes internal-id for non-admin users"
  [result ctx]
  (let [user (user-info (:req ctx))]
    (if (admin-user? user))
      result
      (update result :internal-id nil)))

(defn- update-owner
  "Updates created_by with accessing user's id"
  [args ctx]
  (let [user (user-info (:req ctx))]
    (if (end-user? user)
      (assoc args :created_by (:id user))
      args)))

;; Multiple signal function can be specified as a vector.

(def example-config
  {:signals {:all [check-user-auth check-user-role]
             :users {:query {:pre end-user-access
                             :post hide-internal-id}
                     :create {:pre update-owner}
                     :update {:pre update-owner}}}})

Notes:

• :all can be used at each level of signal map to run signal functions across all tables, all operations for a table, or both timing for a specific operation.