Application Context

The application context passed to your field resolvers is normally set by the initial call to com.walmartlabs.lacinia/execute. Lacinia uses the context for its own book-keeping (the keys it places into the map are namespaced to avoid collisions) but otherwise the same map is passed to all field resolvers.

In specific cases, it is useful to allow a field resolver to modify the application context, with the change exposed just to the fields nested below, but to any depth.

For example, in this query:

{
  products(search: "fuzzy") {
    category {
      name
      product {
        upc
        name
        highlighted_name
      }
   }
}

Here, the search term is provided to the products field, but is again needed by the highlighted_name field, to highlight the parts of the name that match the search term.

The resolver for the products field can communicate this information “down tree” to the resolver for the highlighted_name field, by using the com.walmartlabs.lacinia.resolve/with-context function.

(require '[com.walmartlabs.lacina.resolve :as resolve])

(defn resolve-products
  [_ args _]
  (let [search-term (:search args)]
    (-> (perform-product-search args)
        (resolve/with-context {::search-term search-term}))))

(defn resolve-highlighted-name
  [context _ product]
  (let [{:keys [::search-term]} context]
    (-> product :name (add-highlight search-term))))

The map provided to with-context will be merged into the application context before any nested resolvers are invoked. In this way, the new key, ::search-term, is only present in the context for field resolvers below products field.

Some field resolvers returns lists of values; the entire list can be wrapped in this way OR individual values within the list may be wrapped.

Tip

Remember that query execution is top to bottom, then the final result map is assembled from the leaves back up to the roots.

When using lacinia-pedestal, the default behavior is to capture the Ring request map and supply it in the application context under the :request key.