Now that we have the listings GraphQL query field established, let's update the resolver function for this field such that it will retrieve a list of listings from the "listings" collection from our database when queried.

listings typeDefs#

We'll first update the GraphQL type definitions of the listings field before implementing the resolver function.

The listings field will have three input arguments:

• It will have a filter argument that will be of an Enum type we'll create shortly called ListingsFilter. When this filter argument is passed in, it'll allow the field to return a list of listings that are sorted based on the filter applied.

• It will have a limit argument which is to be an integer to determine the number of listings to return for a certain query.

• It'll have a page argument of type integer which is to determine the page (or in other words the sub-list) of listings to be returned.

When the listings field is to be resolved, it'll return a Listings object type we've set up before which contains the total number of items returned and the list of items returned.

  type Query {

    authUrl: String!

    user(id: ID!): User!

    listing(id: ID!): Listing!

    listings(filter: ListingsFilter!, limit: Int!, page: Int!): Listings!

  }

Next, we'll set up the GraphQL type definition for the ListingsFilter Enum. There are to kinds of filters we'll allow the client to specify for how it wants the collection of listings to be returned in this field. We'll have a filter that will allow the client to query listings from the lowest price to the highest price and we can call the Enum value for this "PRICE_LOW_TO_HIGH". Similarly, the client will also be able to pass in a filter value that will filter listings from the highest price to the lowest. We'll call this filter Enum value "PRICE_HIGH_TO_LOW".

  enum ListingsFilter {

    PRICE_LOW_TO_HIGH

    PRICE_HIGH_TO_LOW

  }

listings() resolver#

With the GraphQL type definition for the root-level listings field prepared, we'll now look to update the accompanying resolver function. The first thing we'll do is define the TypeScript type definitions of the expected arguments and return value of the listings() function resolver. We'll define these TypeScript types in the src/graphql/resolvers/Listing/types.ts file.

We'll first define the shape of arguments that can be passed into the listings field and we'll establish this in an interface we'll call ListingsArgs. There are three current expected arguments - filter, limit, and page. limit and page are to be number's while filter is to be a defined set of values or in other words an Enum. So we'll declare a string-based TypeScript Enum, labeled ListingsFilter, to reflect the different listing filters that can be applied - "PRICE_LOW_TO_HIGH" and "PRICE_HIGH_TO_LOW".

export enum ListingsFilter {

  PRICE_LOW_TO_HIGH = "PRICE_LOW_TO_HIGH",

  PRICE_HIGH_TO_LOW = "PRICE_HIGH_TO_LOW"

}

export interface ListingsArgs {

  filter: ListingsFilter;

  limit: number;

  page: number;

}

We'll then create the shape of the data that is expected to be returned from the listings() resolver function. We'll call this interface ListingsData and it is to have a total field of type number and a result field which is to be a list of listings. We have the definition of a single Listing document defined in our src/lib/types.ts types file so we'll import it and use it within the ListingsData interface.

With these changes, the src/graphql/resolvers/Listing/types.ts file will now look like the following:

import { Booking, Listing } from "../../../lib/types";

export enum ListingsFilter {

  PRICE_LOW_TO_HIGH = "PRICE_LOW_TO_HIGH",

  PRICE_HIGH_TO_LOW = "PRICE_HIGH_TO_LOW"

}

export interface ListingArgs {

  id: string;

}

export interface ListingBookingsArgs {

  limit: number;

  page: number;

}

export interface ListingBookingsData {

  total: number;

  result: Booking[];

}

export interface ListingsArgs {

  filter: ListingsFilter;

  limit: number;

  page: number;

}

export interface ListingsData {

  total: number;

  result: Listing[];

}

We'll now head over to the listingResolvers map file (src/graphql/resolvers/Listing/index.ts) and look to create the resolver functionality for the listings field. We'll import the ListingsArgs and ListingsData interfaces from the adjacent types file and we'll state the arguments and expected return statement for the listings() resolver. In the listing() resolver function, the root object argument will be undefined. The resolver will expect a filter, limit, and page arguments. We'll need access to the db object available in context and when the resolver function is completed successfully, it'll return a Promise that when resolved will resolve to an object that conforms to the ListingsData shape.