# Field Guard

You can define `guard` to field of an `Object`. This permit to add checks before run the code logic of the field.
`Guard` are made of rules that you need to define before. A rule is a structure which implement the trait `Guard`.

```rust
#[derive(Eq, PartialEq, Copy, Clone)]
enum Role {
    Admin,
    Guest,
}

struct RoleGuard {
    role: Role,
}

#[async_trait::async_trait]
impl Guard for RoleGuard {
    async fn check(&self, ctx: &Context<'_>) -> Result<()> {
        if ctx.data_opt::<Role>() == Some(&self.role) {
            Ok(())
        } else {
            Err("Forbidden".into())
        }
    }
}
```

Once you have defined your rule you can use it in the `guard` field attribute.
This attribute support 4 operators to create complex rules :

- `and` : perform a `and` operation between two rules. (If one rule return an error the `and` operator will return the error. If both rules return a error it's the first one that will be returned).

- `or` : perform a `and` operation between two rules. (If both rules return an error the error returned is the first one)

- `chain` : take a set of rules and run them until one return an error or return `Ok()` if all rules pass.

- `race` : take a set of rules and run them until one return `Ok()` if they all fail it return the last error.

```rust
#[derive(SimpleObject)]
struct Query {
    #[graphql(guard(RoleGuard(role = "Role::Admin")))]
    value: i32,
    #[graphql(guard(and(
        RoleGuard(role = "Role::Admin"),
        UserGuard(username = r#""test""#)
    )))]
    value2: i32,
    #[graphql(guard(or(
        RoleGuard(role = "Role::Admin"),
        UserGuard(username = r#""test""#)
    )))]
    value3: i32,
    #[graphql(guard(chain(
        RoleGuard(role = "Role::Admin"),
        UserGuard(username = r#""test""#),
        AgeGuard(age = r#"21"#)
    )))]
    value4: i32,
    #[graphql(guard(race(
        RoleGuard(role = "Role::Admin"),
        UserGuard(username = r#""test""#),
        AgeGuard(age = r#"21"#)
    )))]
    value5: i32,
}
```