Formatting a Date

This is a common scenario: you have a Date or Timestamp value and it needs to be in a specific format in the result map.

In this example, the field resolver will extract the key from the container’s resolved value, and format it:

(fn [context args resolved-value]
    (->> resolved-value
         (format "%tm-%<td-%<tY")))

This example is tied to a specific key (:updated-at) and a specific format. A resolver factory could be used to make this a more general pattern.

Accessing a Java Instance Method

In some cases, you may find yourself exposing JavaBeans as schema objects. This fights against the grain of Lacinia, which expects schema objects to be Clojure maps.

It would be tedious to write a custom field resolver function for each and every Java instance method that needs to be invoked. Instead, we can use a resolver factory.

(defn ^:private make-accessor
  (let [method-name (name method-sym)
        arg-types (make-array Class 0)
        args (make-array Object 0)]
    (fn [value]
      (let [c (class value)
            method (.getMethod c method-name arg-types)]
        (.invoke method value args)))))

(defn resolve-method
  (let [f (make-accessor method-sym)]
    (fn [_ _ value]
      (f value))))

This won’t be the most efficient approach, since it has to lookup a method on each use and then invoke that method using Java reflection, but may be suitable for light use, or as the basis for a more efficient implementation.


More examples forthcoming.