Resolvers

There are multiple ways that a graphQL type can be bound to a Go struct that allows for many usecases.

Bind directly to struct field names

This is the most common use case where the names of the fields on the Go struct match the names of the fields in the graphQL type. If a Go struct field is unexported, it will not be bound to the graphQL type.

type Car struct {
    Make string
    Model string
    Color string
    OdometerReading int
}

And then in your graphQL schema:

type Car {
    make: String!
    model: String!
    color: String!
    odometerReading: Int!
}

And in the gqlgen config file:

models:
    Car:
        model: github.com/my/app/models.Car

In this case, each field in the graphQL type will be bound to the respective field on the go struct ignoring the case of the fields

Bind to a method name

This is also very common use case that comes up where we want to bind a graphQL field to a Go struct method

type Person struct {
    Name string
}

type Car struct {
    Make string
    Model string
    Color string
    OwnerID *string
    OdometerReading int
}

func (c *Car) Owner() (*Person) {
    // get the car owner
    //....
    return owner
}

And then in your graphQL schema:

type Car {
    make: String!
    model: String!
    color: String!
    odometerReading: Int!
    owner: Person
}

And in the gqlgen config file:

models:
    Car:
        model: github.com/my/app/models.Car
    Person:
        model: github.com/my/app/models.Person

Here, we see that there is a method on car with the name Owner, thus the Owner function will be called if a graphQL request includes that field to be resolved.

Model methods can optionally take a context as their first argument. If a context is required, the model method will also be run in parallel.

Bind when the field names do not match

There are two ways you can bind to fields when the Go struct and the graphQL type do not match.

The first way is you can bind resolvers to a struct based off of struct tags like the following:

type Car struct {
    Make string
    ShortState string
    LongState string `gqlgen:"state"`
    Model string
    Color string
    OdometerReading int
}

And then in your graphQL schema:

type Car {
    make: String!
    model: String!
    state: String!
    color: String!
    odometerReading: Int!
}

And in the gqlgen config file add the line:

struct_tag: gqlgen

models:
    Car:
        model: github.com/my/app/models.Car

Here even though the graphQL type and Go struct have different field names, there is a Go struct tag field on longState that matches and thus state will be bound to LongState.

The second way you can bind fields is by adding a line into the config file such as:

type Car struct {
    Make string
    ShortState string
    LongState string
    Model string
    Color string
    OdometerReading int
}

And then in your graphQL schema:

type Car {
    make: String!
    model: String!
    state: String!
    color: String!
    odometerReading: Int!
}

And in the gqlgen config file add the line:

models:
    Car:
        model: github.com/my/app/models.Car
        fields:
            state:
                fieldName: LongState

Binding to Anonymous or Embedded Structs

All of the rules from above apply to a struct that has an embedded struct. Here is an example

type Truck struct {
    Car

    Is4x4 bool
}

type Car struct {
    Make string
    ShortState string
    LongState string
    Model string
    Color string
    OdometerReading int
}

And then in your graphQL schema:

type Truck {
    make: String!
    model: String!
    state: String!
    color: String!
    odometerReading: Int!
    is4x4: Bool!
}

Here all the fields from the Go struct Car will still be bound to the respective fields in the graphQL schema that match

Embedded structs are a good way to create thin wrappers around data access types an example would be:

type Cat struct {
    db.Cat
    //...
}

func (c *Cat) ID() string {
    // return a custom id based on the db shard and the cat's id
     return fmt.Sprintf("%d:%d", c.Shard, c.Id)
}

Which would correlate with a gqlgen config file of:

models:
    Cat:
        model: github.com/my/app/models.Cat

Binding Priority

If a struct_tags config exists, then struct tag binding has the highest priority over all other types of binding. In all other cases, the first Go struct field found that matches the graphQL type field will be the field that is bound.