GraphQL Resolvers

GraphQL types often have fields who's data has to be loaded from the database, calculated from other fields or just formatted. This is achieved by assigning a resolver to that field.

Lets see how we could load the author of a Post.

export const PostDef = `
type Post implements Node
{
  id: ID!
  author: User!
  text: String!
  created: Date!
}
`;

export const PostTypeDefs = [PostDef, NodeDef, UserDef, DateDef];

@Resolver('Post', 'author', PostTypeDefs)
export class AuthorResolver implements IResolver
{
    public handle(source: Post, args, context, info: GraphQLResolveInfo): User
    {
        return users.find(u => u.id == source.user);
    }
}

In this example we assigned our resolver to the author field of the Post type and we referenced the type defs related to the Post type.

The arguments to @Resolver are:

• type name
• field name
• type def or an array of type defs

For PostTypeDefs we referenced 4 type defs:

• PostDef because it is our target type
• NodeDef because Post implements the Node interface
• UserDef because the author field is of type User
• DateDef because the created field is of type Date

Base resolvers

In most cases you'll want to check user's access rights before resolving a field. To avoid doing this in each individual resolver it is recommended to instead use a base resolver class which could look like this:

export abstract class ResolverBase implements IResolver
{
    public handle(source, args, context, info: GraphQLResolveInfo): any
    {
        // check access...

        return this.resolve(source, args, context, info);
    }

    public abstract resolve(source, args, context, info: GraphQLResolveInfo): any;
}