From a server's point of view, GraphQL unions are similar to interfaces: the only exception is that they don't contain fields on their own.

In Juniper, the graphql_union! has identical syntax to the interface macro, but does not support defining fields. Therefore, the same considerations about using traits, placeholder types, or enums still apply to unions.

If we look at the same examples as in the interfaces chapter, we see the similarities and the tradeoffs:

#[derive(juniper::GraphQLObject)]
struct Human {
    id: String,
    home_planet: String,
}

#[derive(juniper::GraphQLObject)]
struct Droid {
    id: String,
    primary_function: String,
}

trait Character {
    // Downcast methods, each concrete class will need to implement one of these
    fn as_human(&self) -> Option<&Human> { None }
    fn as_droid(&self) -> Option<&Droid> { None }
}

impl Character for Human {
    fn as_human(&self) -> Option<&Human> { Some(&self) }
}

impl Character for Droid {
    fn as_droid(&self) -> Option<&Droid> { Some(&self) }
}

#[juniper::graphql_union]
impl<'a> GraphQLUnion for &'a dyn Character {
    fn resolve(&self) {
        match self {
            Human => self.as_human(),
            Droid => self.as_droid(),
        }
    }
}

# fn main() {}

FIXME: This example does not compile at the moment

# use std::collections::HashMap;
#[derive(juniper::GraphQLObject)]
#[graphql(Context = Database)]
struct Human {
    id: String,
    home_planet: String,
}

#[derive(juniper::GraphQLObject)]
#[graphql(Context = Database)]
struct Droid {
    id: String,
    primary_function: String,
}

struct Database {
    humans: HashMap<String, Human>,
    droids: HashMap<String, Droid>,
}

impl juniper::Context for Database {}

trait Character {
    fn id(&self) -> &str;
}

impl Character for Human {
    fn id(&self) -> &str { self.id.as_str() }
}

impl Character for Droid {
    fn id(&self) -> &str { self.id.as_str() }
}


#[juniper::graphql_union(
    Context = Database
)]
impl<'a> GraphQLUnion for &'a dyn Character {
    fn resolve(&self, context: &Database) {
        match self {
            Human => context.humans.get(self.id()),
            Droid => context.droids.get(self.id()),
        }
    }
}

# fn main() {}

# use std::collections::HashMap;
#[derive(juniper::GraphQLObject)]
#[graphql(Context = Database)]
struct Human {
    id: String,
    home_planet: String,
}

#[derive(juniper::GraphQLObject)]
#[graphql(Context = Database)]
struct Droid {
    id: String,
    primary_function: String,
}

struct Database {
    humans: HashMap<String, Human>,
    droids: HashMap<String, Droid>,
}

impl juniper::Context for Database {}

struct Character {
    id: String,
}

#[juniper::graphql_union(
    Context = Database,
)]
impl GraphQLUnion for Character {
    fn resolve(&self, context: &Database) {
        match self {
            Human => { context.humans.get(&self.id) },
            Droid => { context.droids.get(&self.id) },
        }
    }
}

# fn main() {}

#[derive(juniper::GraphQLObject)]
struct Human {
    id: String,
    home_planet: String,
}

#[derive(juniper::GraphQLObject)]
struct Droid {
    id: String,
    primary_function: String,
}

# #[allow(dead_code)]
enum Character {
    Human(Human),
    Droid(Droid),
}

#[juniper::graphql_union]
impl Character {
    fn resolve(&self) {
        match self {
            Human => { match *self { Character::Human(ref h) => Some(h), _ => None } },
            Droid => { match *self { Character::Droid(ref d) => Some(d), _ => None } },
        }
    }
}

# fn main() {}