Add Field Collection docs

docs/content/reference/field-collection.md

@@ -0,0 +1,57 @@
+---
+title: 'Determining which fields were requested by a query'
+description: How to determine which fields a query requested in a resolver.
+linkTitle: Field Collection
+menu: { main: { parent: 'reference' } }
+---
+
+Often it is useful to know which fields were queried for in a resolver.  Having this information can allow a resolver to only fetch the set of fields required from a data source, rather than over-fetching everything and allowing gqlgen to do the rest.
+
+This process is known as [Field Collection](https://facebook.github.io/graphql/draft/#sec-Field-Collection) — gqlgen automatically does this in order to know which fields should be a part of the response payload.  The set of collected fields does however depend on the type being resolved.  Queries can contain fragments, and resolvers can return interfaces and unions, therefore the set of collected fields cannot be fully determined until the type of the resolved object is known.
+
+Within a resolver, there are several API methods available to query the selected fields.
+
+## CollectAllFields
+
+`CollectAllFields` is the simplest way to get the set of queried fields.  It will return a slice of strings of the field names from the query.  This will be a unique set of fields, and will return all fragment fields, ignoring fragment Type Conditions.
+
+Given the following example query:
+
+```graphql
+query {
+    foo {
+        fieldA
+        ... on Bar {
+            fieldB
+        }
+        ... on Baz {
+            fieldC
+        }
+    }
+}
+```
+
+Calling `CollectAllFields` from a resolver will yield a string slice containing `fieldA`, `fieldB`, and `fieldC`.
+
+## CollectFieldsCtx
+
+`CollectFieldsCtx` is useful in cases where more information on matches is required, or the set of collected fields should match fragment type conditions for a resolved type.  `CollectFieldsCtx` takes a `satisfies` parameter, which should be a slice of strings of types that the resolved type will satisfy.
+
+For example, given the following schema:
+
+```graphql
+interface Shape {
+    area: Float
+}
+type Circle implements Shape {
+    radius: Float
+    area: Float
+}
+union Shapes = Circle
+```
+
+The type `Circle` would satisfy `Circle`, `Shape`, and `Shapes` — these values should be passed to `CollectFieldsCtx` to get the set of collected fields for a resolved `Circle` object.
+
+> Note
+>
+> `CollectFieldsCtx` is just a convenience wrapper around `CollectFields` that calls the later with the selection set automatically passed through from the resolver context.