Using Resolvers

Resolvers are functions that make up the basis for how data is determined in resolving queries in GraphQL APIs. There are two types of resolvers, Field Resolvers and Type Resolvers. Field Resolvers represent the specific data returned within the data key a result payload. Type Resolvers are responsible for taking an Abstract Type (GraphQL Union or Interface types) and returning a string representing the concrete type.

Field Resolvers

Field resolvers are represented by the following function:

async function (parent, args, context, info) {
return data;
}

The majority of the time when Resolvers are mentioned in a GraphQL context it is referring to Field Resolvers. Field Resolvers are responsible for returning the data specified by the GraphQL Schema (or a Promise of the data). Check out the apollo documentation on resolvers, too, it applies here and to GraphQL in general.

With the given schema:

schema {
# denotes the "Root Query Type" as the "Query" type
query: Query
}
# "Query" is the "Root Query Type" as denoted by the schema definition
type Query {
# root field that returns an "Actor"
actor: Actor!
}
type Actor {
name: String!
}

This schema would have two resolvers:

  • One "root field resolver" on the Query type for the actor field
  • One "non-root field resolver" on the Actor type for the name field

Root (Query and Mutation) Field Resolvers

All GraphQL queries start with a "root" field. Root Field Resolvers are either on the root Query type or the root Mutation type depending on whether it's a query or mutation operation, respectively. These root resolvers "kick off" the query and being at the "root" typically have a null parent argument (unless one has been explicitly declared).

A root resolver function for the actor field could be:

async function (parent, args, context, info) {
return {
name: "Meryl Streep"
};
}

Non-Root Field Resolvers

Any type is that not the root Query type or the root Mutation type will be a type that contains non-root field resolvers. In this case they are connected to the graph through some reference to the root Query and Mutation types. Due to the graph-like nature of resolving queries these non-root fields and their resolvers can be referenced multiple times and as such often rely on the parent parameters to understand their connection in the graph.

The non-root field on Actor is name. When any type resolves an Actor type each of the field resolvers receives the parent and is responsible for returning a result for their field.

The name field resolver on the Actor type could look like:

async function (parent, args, context, info) {
return parent.name;
}

This example looks for the name property on the parent. In this case the parent was:

{
"name": "Meryl Streep"
}

The returned value is "Meryl Streep" which matches the return value of String! set by the name field on the Actor type.

Now, to show the flexible nature of resolvers lets expand the the Actor type:

type Actor {
name: String!
favoriteActor: Actor
}

Here we have the favoriteActor of an Actor being another Actor. This demonstrates the how types can reference each other and resolver functions have to be able to return the relevant data whether called from a Root Query or Root Mutation type, or from another type, or the same type itself.

If Tom Hanks was Meryl Streep's favorite actor the favoriteActor resolver could look like:

async function (parent, args, context, info) {
if (parent.name === "Meryl Streep") {
return "Tom Hanks";
}
return null;
}

Only when the parent Actor has a name of "Meryl Streep" do we return "Tom Hanks" in other cases we will return null for all other Actors. This could be quite tedious to manage all the favorite actors from a single resolver so it is usually best practice to have a data source or a well-referenced look-up that can be used. Typically, the parent also has an ID field that can be referenced for lookup also.

Default Field Resolver

It should be noted that GraphQL sets up every field with a default Field Resolver function. In fact it can be imported from the graphql package, too.

import { defaultFieldResolver } from 'graphql';

It will attempt to resolve a field based on the a look up of the field's name on the parent. This typically a desired behavior so often you don't have to write a resolver at all, graphql will do this "out of the box".

There are a couple of other things the default Field Resolver does, check out the source code, it's under 10 lines!

parent parameter (first)

Sometimes called obj, and be named whatever you call the variable, represents the previous returned value in the "graph" of results. As mentioned, in root field resolvers this will typically be null since the query has just begun and has no parent.

arg parameter (second)

When a schema specifies arguments and those values are provided they are available on the field by its arg parameter. Instead of defaulting to "Meryl Streep" what if we had a list of actors required the name to be specified and returned the actor that matched.

type Query {
# an additon of a required `name` argument to look up the actor
actor(name: String): Actor!
}
const actors = [ { name: 'Meryl Streep' }, { name: 'Tom Hanks' } ];
async function (parent, args, context, info) {
return actors.find(actor => actor.name === args.name);
}

And the value for name can be referenced as an argument:

query {
actor(name: "Tom Hanks") {
name
}
}

The source of this actors data is a bit hand-wavy, there are ways of accessing these data sources (typically from the context parameter), but what is important is how arguments are passed to the field and its resolver via the args parameter.

context parameter (third)

This is the "global" bucket that is accessible in every resolver. It's usually an object. It's typically a good place to put data sources that can combined with the parent and args can be used to lookup. graphql-mocks has an opinion approach with a managed context to provide conventions and assist in threading the common use cases.

For example, dependencies that are passed to the graphql-mocks GraphQL Handler can reliably be pulled from context using the extractDependencies function. There are other helpful ways that context can be used with graphql-mocks, too!

info parameter (fourth)

This is mostly a holder of meta and GraphQL information. It's not usually needed but does have some handy insights into the query, AST, and access to the GraphQL schema.

Type Resolvers

While covered less the resolveType Type Resolver function on the GraphQLInterfaceType and GraphQLUnionType play an important role in resolving GraphQL queries. See the Apollo documentation for additional examples.

Type Resolver functions have the following signature:

async function (value, context, info, abstractType) {
return 'ResolvedTypeName';
}

The objective for a Type Resolver is given a "value", the object in question, to determine which discrete type is represented. The return value of a Type Resolver function is a string representing the concrete type name, ie: Dog or Cat in the following example for the Pet abstract type.

Here's an example:

schema {
query: Query
}
type Query {
pet: Pet!
}
union Pet = Cat | Dog
type Cat {
name: String!
}
type Dog {
name: String!
likesBones: Boolean!
}

And our Type Resolver:

async function (value, context, info, abstractType) {
if ('likesBones' in value) {
return 'Dog';
} else {
return 'Cat';
}
}

We are given the concrete object in question and are responsible for determining which type of Pet it is: either a Pet is a Dog or a Cat. In this contrived example the likesBones property is checked to exist and if it does we assume it's a Dog. This is something to consider when working with Abstract Types (Unions and Interfaces) is how the underlying concrete types will be identified. It is usually to leave a concrete type identifier __typename on the concrete type which would allow the default type resolver to automatically use the value.

value parameter (first)

The ambiguous object in question that needs to be identified to a concrete type

context parameter (second)

The same context global object as represented in Field Resolvers.

info parameter (third)

Represents the meta details and GraphQL information, is usually only needed in exceptional cases.

abstractType parameter (fourth)

abstractType is either a GraphQLInterfaceType or GraphQLUnionType and the instance provides the details of the abstract type that is being resolved down to a concrete type.

Default Type Resolver

Similar to Field Resolvers, Type Resolvers also have a default resolver that exists on the resolveType of the Interface and Union types.

import { defaultTypeResolver } from 'graphql';

The default behavior the defaultTypeResolver is to check the object for the __typename field and return the value if it exists, or less commonly to check isTypeOf on each on each possible type with the object being coerced. Check out the source code for more details.

Organizing Resolvers for Executing GraphQL queries

This page focuses on the Resolver functions themselves but hasn't shown how they are actually applied to the Fields and Abstract Types that they are responsible for resolving. On their own they are "just functions" and need a way of being assigned. This is the role of a Resolver Map which organized these Resolver Functions so that they can be applied to a GraphQLSchema and executed against queries and mutations, check out the Resolver Map to understand the next step in applying Resolver functions.

Extending Resolvers with Resolver Wrappers

The next section will introduce Resolver Wrappers from this library and how they can be used to allow Resolvers functions to be extended.

Additional Resources