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< h1 id = "implicit-and-explicit-null" > < a class = "header" href = "#implicit-and-explicit-null" > Implicit and explicit < code > null< / code > < / a > < / h1 >
< blockquote >
< p > < a href = "https://graphql.org" > GraphQL< / a > has two semantically different ways to represent the lack of a value:< / p >
< ul >
< li > Explicitly providing the literal value: < strong > null< / strong > .< / li >
< li > Implicitly not providing a value at all.< / li >
< / ul >
< / blockquote >
< p > There are two ways that a client can submit a < a href = "https://spec.graphql.org/October2021#sec-Null-Value" > < code > null< / code > value< / a > as an < a href = "https://spec.graphql.org/October2021#sec-Language.Arguments" > argument< / a > or a < a href = "https://spec.graphql.org/October2021#sec-Language.Fields" > field< / a > in a < a href = "https://graphql.org" > GraphQL< / a > query:< / p >
< ol >
< li > Either use an explicit < code > null< / code > literal:
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< pre > < code class = "language-graphql" > {
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field(arg: null)
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}
< / code > < / pre >
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< / li >
< li > Or simply omit the < a href = "https://spec.graphql.org/October2021#sec-Language.Arguments" > argument< / a > , so the implicit default < code > null< / code > value kicks in:
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< pre > < code class = "language-graphql" > {
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field
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}
< / code > < / pre >
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< / li >
< / ol >
< p > There are some situations where it's useful to know which one exactly has been provided.< / p >
< p > For example, let's say we have a function that allows users to perform a "patch" operation on themselves. Let's say our users can optionally have favorite and least favorite numbers, and the input for that might look like this:< / p >
< pre > < pre class = "playground" > < code class = "language-rust edition2021" > /// Updates user attributes. Fields that are [`None`] are left as-is.
struct UserPatch {
/// If [`Some`], updates the user's favorite number.
favorite_number: Option< Option< i32> > ,
/// If [`Some`], updates the user's least favorite number.
least_favorite_number: Option< Option< i32> > ,
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}
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< span class = "boring" >
< / span > < span class = "boring" > fn main() {}< / span > < / code > < / pre > < / pre >
< p > To set a user's favorite number to 7, we would set < code > favorite_number< / code > to < code > Some(Some(7))< / code > . In < a href = "https://graphql.org" > GraphQL< / a > , that might look like this:< / p >
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< pre > < code class = "language-graphql" > mutation { patchUser(patch: { favoriteNumber: 7 }) }
< / code > < / pre >
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< p > To unset the user's favorite number, we would set < code > favorite_number< / code > to < code > Some(None)< / code > . In < a href = "https://graphql.org" > GraphQL< / a > , that might look like this:< / p >
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< pre > < code class = "language-graphql" > mutation { patchUser(patch: { favoriteNumber: null }) }
< / code > < / pre >
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< p > And if we want to leave the user's favorite number alone, just set it to < code > None< / code > . In < a href = "https://graphql.org" > GraphQL< / a > , that might look like this:< / p >
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< pre > < code class = "language-graphql" > mutation { patchUser(patch: {}) }
< / code > < / pre >
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< p > The last two cases rely on being able to distinguish between < a href = "https://spec.graphql.org/October2021#sel-EAFdRDHAAEJDAoBxzT" > explicit and implicit < code > null< / code > < / a > .< / p >
< p > Unfortunately, plain < code > Option< / code > is not capable to distinguish them. That's why in < a href = "https://docs.rs/juniper" > Juniper< / a > , this can be done using the < a href = "https://docs.rs/juniper/latest/juniper/enum.Nullable.html" > < code > Nullable< / code > < / a > type:< / p >
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< pre > < pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > extern crate juniper;
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< / span > use juniper::{graphql_object, FieldResult, GraphQLInputObject, Nullable};
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#[derive(GraphQLInputObject)]
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struct UserPatchInput {
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favorite_number: Nullable< i32> ,
least_favorite_number: Nullable< i32> ,
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}
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impl From< UserPatchInput> for UserPatch {
fn from(input: UserPatchInput) -> Self {
Self {
// The `explicit()` function transforms the `Nullable` into an
// `Option< Option< T> > ` as expected by the business logic layer.
favorite_number: input.favorite_number.explicit(),
least_favorite_number: input.least_favorite_number.explicit(),
}
}
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}
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< span class = "boring" > struct UserPatch {
< / span > < span class = "boring" > favorite_number: Option< Option< i32> > ,
< / span > < span class = "boring" > least_favorite_number: Option< Option< i32> > ,
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< / span > < span class = "boring" > }
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< / span > < span class = "boring" >
< / span > < span class = "boring" > struct Session;
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< / span > < span class = "boring" > impl Session {
< / span > < span class = "boring" > fn patch_user(& self, _patch: UserPatch) -> FieldResult< ()> { Ok(()) }
< / span > < span class = "boring" > }
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< / span > < span class = "boring" >
< / span > struct Context {
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session: Session,
}
impl juniper::Context for Context {}
struct Mutation;
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#[graphql_object]
#[graphql(context = Context)]
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impl Mutation {
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fn patch_user(patch: UserPatchInput, ctx: & Context) -> FieldResult< bool> {
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ctx.session.patch_user(patch.into())?;
Ok(true)
}
}
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< span class = "boring" >
< / span > < span class = "boring" > fn main() {}< / span > < / code > < / pre > < / pre >
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< / nav >
< / div >
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< script >
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window.playground_copyable = true;
< / script >
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< script src = "../elasticlunr.min.js" > < / script >
< script src = "../mark.min.js" > < / script >
< script src = "../searcher.js" > < / script >
< script src = "../clipboard.min.js" > < / script >
< script src = "../highlight.js" > < / script >
< script src = "../book.js" > < / script >
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<!-- Custom JS scripts -->
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< / div >
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< / body >
< / html >
