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 — 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:
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:
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 aroundCollectFields
that calls the later with the selection set automatically passed through from the resolver context.
Practical example
Say we have the following GraphQL query
query {
flowBlocks {
id
block {
id
title
type
choices {
id
title
description
slug
}
}
}
}
We don’t want to overfetch our database so we want to know which field are requested. Here is an example which get’s all requested field as convenient string slice, which can be easily checked.
func GetPreloads(ctx context.Context) []string {
return GetNestedPreloads(
graphql.GetOperationContext(ctx),
graphql.CollectFieldsCtx(ctx, nil),
"",
)
}
func GetNestedPreloads(ctx *graphql.OperationContext, fields []graphql.CollectedField, prefix string) (preloads []string) {
for _, column := range fields {
prefixColumn := GetPreloadString(prefix, column.Name)
preloads = append(preloads, prefixColumn)
preloads = append(preloads, GetNestedPreloads(ctx, graphql.CollectFields(ctx, column.Selections, nil), prefixColumn)...)
}
return
}
func GetPreloadString(prefix, name string) string {
if len(prefix) > 0 {
return prefix + "." + name
}
return name
}
So if we call these helpers in our resolver:
func (r *queryResolver) FlowBlocks(ctx context.Context) ([]*FlowBlock, error) {
preloads := GetPreloads(ctx)
it will result in the following string slice:
["id", "block", "block.id", "block.title", "block.type", "block.choices", "block.choices.id", "block.choices.title", "block.choices.description", "block.choices.slug"]