Add a field

This guide adds bestReview to the Movie type: the highest rated review of the movie, or null when it has none. Movie already carries its reviews, so this is pure Presentation work, a resolver on the type. For a field that needs new data, add a query to src/Application first and send it through Mediator the way the existing resolvers do.

Add the resolver to src/Presentation/GraphQL/Types/MovieType.cs. The [Parent] attribute hands it the movie being resolved, and the XML summary becomes the field’s description in the schema.

using HotChocolate;
using HotChocolate.Types;

[ObjectType<Movie>]
public static partial class MovieType
{
    static partial void Configure(IObjectTypeDescriptor<Movie> descriptor)
    {
        _ = descriptor.Field(m => m.Id);
        _ = descriptor.Field(m => m.Title).Type<NonNullType<StringType>>();
        _ = descriptor.Field(m => m.Reviews).Type<ListType<NonNullType<ObjectType<Review>>>>();
    }

    /// <summary>
    /// The highest rated review of the movie, or null when it has no reviews.
    /// </summary>
    public static Review? GetBestReview([Parent] Movie movie)
    {
        return movie.Reviews.MaxBy(r => r.Stars);
    }
}

Hot Chocolate drops the Get prefix and camel-cases the rest, so the field is bestReview.

Run the tests and watch one fail.
Terminal window
```
dotnet test
```
```
The schema has changed. If the change is intended, copy the output of /graphql?sdl into
tests/Presentation.Tests.Integration/GraphQL/Schema/schema.graphql.
```
That’s the schema snapshot test doing its job. It’s the one thing standing between you and a client that breaks on Tuesday.

The change is intended, so update the snapshot. Run the API and save the SDL over the file.

dotnet run --project ./src/Presentation
curl "http://localhost:5032/graphql?sdl" > tests/Presentation.Tests.Integration/GraphQL/Schema/schema.graphql

The diff is the one field, described:

"A movie that can be reviewed."
type Movie {
  "The highest rated review of the movie, or null when it has no reviews."
  bestReview: Review
  "The unique identifier of the movie."
  id: UUID!
  "The title of the movie."
  title: String!
  "The reviews written about the movie."
  reviews: [Review!]
}

Teach the test models about it. Add the property to the Movie class in tests/Presentation.Tests.Integration/GraphQL/Movies/MovieData.cs.
```
[JsonPropertyName("bestReview")]
public Review? BestReview { get; set; }
```

Test it in tests/Presentation.Tests.Integration/GraphQL/Movies/MovieTests.cs. Pick a movie that has reviews and check the field agrees with the list.

[Fact]
public async Task Movie_ShouldReturn_BestReview()
{
    // Arrange
    using var client = Application.CreateClient();
    using var setupResponse = await client.GetAsync("/graphql?query={movies(first:50){nodes{id,reviews{id}}}}");
    var setupResult = (await setupResponse.Content.ReadAsStringAsync()).Deserialize<GraphData>();
    var movieId = setupResult.Data.Movies.Nodes.First(m => m.Reviews.Length > 0).Id;

    // Act
    using var response = await client.GetAsync($"/graphql?query={{movie(id:\"{movieId}\"){{reviews{{stars}},bestReview{{stars}}}}}}");
    var result = (await response.Content.ReadAsStringAsync()).Deserialize<GraphData>();

    // Assert
    response.StatusCode.ShouldBe(HttpStatusCode.OK);

    result.Data.Movie.BestReview.ShouldNotBeNull().Stars.ShouldBe(result.Data.Movie.Reviews.Max(r => r.Stars));
}

Run it all again, then ask Nitro.

dotnet test

{
  movie(id: "e159975e-4e6a-1ffd-713e-1eb9d8d4a593") {
    title
    bestReview { stars author { firstName lastName } }
  }
}

{ "data": { "movie": { "title": "Assimilated disintermediate adapter", "bestReview": { "stars": 5, "author": { "firstName": "Jana", "lastName": "Auer" } } } } }

Jana gave it five stars. Jana’s average is 2.6, so this is high praise.

Fields that need their own data

MovieSummary.reviews is the pattern to copy when a field has to fetch something. It’s a resolver that takes an IReviewsByMovieIdDataLoader, and the DataLoader in GraphQL/DataLoaders collects every movie id in the response and sends one GetReviewsForMoviesQuery for the lot. A page of 50 movies with their reviews costs three queries, not 52. Write the batch query in Application, the DataLoader in Presentation, and let Hot Chocolate do the collecting.