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Fields

GraphQL supports arguments on query fields. We saw this already with the SchemaBuilder.FromObject() helper method. It created a field with an id argument to select a single item by id. Of course you can create fields with your own arguments to expand the functionality of you GraphQL APi.

Adding a field with an argument​

Arguments are defined using an anonymous object type when defining the field. Let's build the id example ourselves.

// empty schema
var schema = new SchemaProvider<DemoContext>();
schema.AddType<Person>("Person", "Information about a person");

// add our root-level query field with an argument
schema.Query().AddField(
    "person",
    new {
        id = ArgumentHelper.Required<int>()
    },
    (ctx, args) => ctx.People.FirstOrDefault(p => p.Id == args.id),
    "Fetch person by ID"
);

Let's break this down.

  • Each property of the anonymous object will be an argument on the GraphQL field. In this case we have 1 argument named id
  • ArgumentHelper.Required<T>() lets EntityGraphQL know that the argument is required and will error if it is not supplied in the query
  • The field resolve expression now has 2 parameters, the original context and the anonymous object type which at execution time will contain the argument values
  • We use the args.id to filter the list of people to a single person that will be returned

It is worth noting that the GraphQL name of the argument will be that of the property on the anonymous type. In GraphQL arguments typically start with lower case hence we use id. If you used Id as the argument name in the anonymous object you will need to use Id in queries (GraphQL is case sensitive).

Required arguments​

As you see above to make an argument required use the ArgumentHelper.Required<T>() method.

See Validation for further information.

Optional arguments​

If you want the argument to be optional just set the field to a typed null value.

schema.Query().AddField(
    "people",
    new {
        firstName = (string)null
    },
    (ctx, args) => !string.IsNullOrEmpty(args.firstName) ? ctx.People.Where(p => p.FirstName == args.firstName) : ctx.People,
    "List of people optionally filtered by a first name"
);

If the argument is not supplied with a value in the query args.firstName will be null. We can use that to optionally filter the data.

Default values​

In GraphQL field arguments can have default values. In EntityGraphQL you just set the default value in the anonymous object create. If fact that is what we did above with null.

schema.Query().AddField(
    "people",
    new {
        deleted = false // default value
    },
    (ctx, args) => ctx.People.Where(p => p.IsDeleted == args.deleted),
    "List all active people. Optional list all deleted people"
);

Using methods as fields​

EntityGraphQL can map methods on your classes to query fields using the GraphQLFieldAttribute. When using SchemaBuilder or SchemaType.AddAllFields() EntityGraphQL will add any methods that have the GraphQLFieldAttribute as a field on that type. The parameters of the method will become the GraphQL field arguments. Mapping method parameters to GraphQL arguments follow the same rules as mutations and subscription methods.

Using [GraphQLInputType] on a parameter will include the parameter as an argument and use the type as an input type. [GraphQLArguments] will flatten the properties of that parameter type into many arguments in the schema.

When looking for a methods parameters, EntityGraphQL will

  1. First all scalar / non-complex types will be added as arguments in the schema.

  2. If parameter type or enum type is already in the schema it will be added at an argument.

  3. Any argument or type with GraphQLInputTypeAttribute will be added to the schema as an InputType

  4. Any argument or type with GraphQLArgumentsAttribute found will have the types properties added as schema arguments.

  5. If no attributes are found it will assume they are services and not add them to the schema. I.e. Label your parameters with the attributes or add the type to the schema beforehand.

Note about method execution with EF​

Let's look at an example.

public class DemoContext : DbContext
{
    [Description("Collection of Movies")]
    public DbSet<Movie> Movies { get; set; }
    [Description("Collection of Peoples")]
    public DbSet<Person> People { get; set; }
    [Description("Collection of Actors")]
    public DbSet<Actor> Actors { get; set; }
}

public class Movie
{
    public uint Id { get; set; }
    public string Name { get; set; }

    // other fields hidden

    public virtual DateTime Released { get; set; }
    public virtual List<Actor> Actors { get; set; }
    public virtual Person Director { get; set; }

    [GraphQLField]
    public uint DirectorAgeAtRelease => (uint)((Released - Director.Dob).Days / 365);
}

var schema = new SchemaProvider<DemoContext>();

DirectorAgeAtRelease will become a field directorAgeAtRelease on the GraphQL type Movie with no arguments. If you query this field like

{
  movies {
    name
    directorAgeAtRelease
  }
}

It will generate the follow expression

(ctx) => ctx.Movies.Select(m => new {
    name = m.Name,
    directorAgeAtRelease = m.DirectorAgeAtRelease()
})

If you are using EF, it does not know how to translate your method and will try to execute this method after fetching data. This issue is the method relies on the Director property on Movie which EF doesn't know and did not fetch so you will not get the expected result. There are a few solutions to this.

Lazy Loading​

One solution is Microsoft.EntityFrameworkCore.Proxies to lazy load the properties, which comes with its own tradeoffs. If you install the Nuget package and follow the instructions for using lazy loading proxies these methods will work.

EntityFrameworkCore.Projectables Library​

Another solution is EntityFrameworkCore.Projectables (Or Expressionify) which uses source generators to generate an EF compatible expression.

// configure projectables
services.AddDbContext<DemoContext>(opt =>
    opt.UseProjectables()
);

// add the attributes to your methods
[GraphQLField]
[Projectable]
public uint DirectorAgeAtRelease => (uint)((Released - Director.Dob).Days / 365);

Now the expression will be translated into expression that EF will handle. See EntityFrameworkCore.Projectables documentation for more information. Or your preferred library.

These libraries require the methods by expression - For example EntityFrameworkCore.Projectables will not work for a method like below, whereas lazy loading will.

[GraphQLField]
[Projectable] // will fail as it can't generate an expression - does work with lazy loading
public uint[] AgesOfActorsAtRelease()
{
    var ages = new List<uint>();
    foreach (var actor in Actors)
    {
        ages.Add((uint)((Released - actor.Person.Dob).Days / 365));
    }
    return ages.ToArray();
}

Helper methods​

WhereWhen()​

The conditional statement we saw above - !string.IsNullOrEmpty(args.firstName) ? ctx.People.Where(p => p.FirstName == args.firstName) : ctx.People - can start to get messy when you have multiple optional arguments. It is common to have multiple arguments on fields that return a list of items to filter for different uses. To aid this EntityGraphQL has a helper method WhereWhen() that only applies a Where() method if a given statement returns true.

schema.Query().AddField(
    "people",
    new {
        // multiple optional arguments
        firstName = (string)null,
        lastName = (string)null
    },
    (ctx, args) => ctx.People
        .WhereWhen(p => p.FistName == args.firstName, !string.IsNullOrEmpty(args.firstName))
        .WhereWhen(p => p.LastName == args.lastName, !string.IsNullOrEmpty(args.lastName)),
    "List of people optionally filtered by a first and/or last name"
);

Take(int?)​

Only apply the Take() method if the argument has a value.

schema.Query().AddField("Field", new { limit = (int?)null }, (db, p) => db.Entity.Take(p.limit), "description");