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< h1 class = "menu-title" > Juniper - GraphQL Server for Rust< / h1 >
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< a class = "header" href = "#scalars" id = "scalars" > < h1 > Scalars< / h1 > < / a >
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< p > Scalars are the primitive types at the leaves of a GraphQL query: numbers,
strings, and booleans. You can create custom scalars to other primitive values,
but this often requires coordination with the client library intended to consume
the API you're building.< / p >
< p > Since any value going over the wire is eventually transformed into JSON, you're
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also limited in the data types you can use.< / p >
< p > There are two ways to define custom scalars.< / p >
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< ul >
< li > For simple scalars that just wrap a primitive type, you can use the newtype pattern with
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a custom derive.< / li >
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< li > For more advanced use cases with custom validation, you can use
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the < code > graphql_scalar< / code > proc macro.< / li >
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< / ul >
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< a class = "header" href = "#built-in-scalars" id = "built-in-scalars" > < h2 > Built-in scalars< / h2 > < / a >
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< p > Juniper has built-in support for:< / p >
< ul >
< li > < code > i32< / code > as < code > Int< / code > < / li >
< li > < code > f64< / code > as < code > Float< / code > < / li >
< li > < code > String< / code > and < code > & str< / code > as < code > String< / code > < / li >
< li > < code > bool< / code > as < code > Boolean< / code > < / li >
< li > < code > juniper::ID< / code > as < code > ID< / code > . This type is defined < a href = "http://facebook.github.io/graphql/#sec-ID" > in the
spec< / a > as a type that is serialized
as a string but can be parsed from both a string and an integer.< / li >
< / ul >
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< p > Note that there is no built-in support for < code > i64< / code > /< code > u64< / code > , as the GraphQL spec < a href = "https://spec.graphql.org/June2018/#sec-Int" > doesn't define any built-in scalars for < code > i64< / code > /< code > u64< / code > by default< / a > . You may wish to leverage a < a href = "#custom-scalars" > custom GraphQL scalar< / a > in your schema to support them.< / p >
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< p > < strong > Third party types< / strong > :< / p >
< p > Juniper has built-in support for a few additional types from common third party
crates. They are enabled via features that are on by default.< / p >
< ul >
< li > uuid::Uuid< / li >
< li > chrono::DateTime< / li >
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< li > time::{Date, OffsetDateTime, PrimitiveDateTime, Time, UtcOffset}< / li >
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< li > url::Url< / li >
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< li > bson::oid::ObjectId< / li >
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< / ul >
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< a class = "header" href = "#newtype-pattern" id = "newtype-pattern" > < h2 > newtype pattern< / h2 > < / a >
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< p > Often, you might need a custom scalar that just wraps an existing type.< / p >
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< p > This can be done with the newtype pattern and a custom derive, similar to how
serde supports this pattern with < code > #[serde(transparent)]< / code > .< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > # extern crate juniper;
#[derive(juniper::GraphQLScalarValue)]
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pub struct UserId(i32);
#[derive(juniper::GraphQLObject)]
struct User {
id: UserId,
}
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# fn main() {}
< / code > < / pre > < / pre >
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< p > That's it, you can now user < code > UserId< / code > in your schema.< / p >
< p > The macro also allows for more customization:< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > # extern crate juniper;
/// You can use a doc comment to specify a description.
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#[derive(juniper::GraphQLScalarValue)]
#[graphql(
transparent,
// Overwrite the GraphQL type name.
name = " MyUserId" ,
// Specify a custom description.
// A description in the attribute will overwrite a doc comment.
description = " My user id description" ,
)]
pub struct UserId(i32);
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# fn main() {}
< / code > < / pre > < / pre >
< a class = "header" href = "#custom-scalars" id = "custom-scalars" > < h2 > Custom scalars< / h2 > < / a >
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< p > For more complex situations where you also need custom parsing or validation,
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you can use the < code > graphql_scalar< / code > proc macro.< / p >
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< p > Typically, you represent your custom scalars as strings.< / p >
< p > The example below implements a custom scalar for a custom < code > Date< / code > type.< / p >
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< p > Note: juniper already has built-in support for the < code > chrono::DateTime< / code > type
via < code > chrono< / code > feature, which is enabled by default and should be used for this
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purpose.< / p >
< p > The example below is used just for illustration.< / p >
< p > < strong > Note< / strong > : the example assumes that the < code > Date< / code > type implements
< code > std::fmt::Display< / code > and < code > std::str::FromStr< / code > .< / p >
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< pre > < pre class = "playpen" > < code class = "language-rust" > # extern crate juniper;
# mod date {
# pub struct Date;
# impl std::str::FromStr for Date {
# type Err = String; fn from_str(_value: & str) -> Result< Self, Self::Err> { unimplemented!() }
# }
# // And we define how to represent date as a string.
# impl std::fmt::Display for Date {
# fn fmt(& self, _f: & mut std::fmt::Formatter) -> std::fmt::Result {
# unimplemented!()
# }
# }
# }
#
use juniper::{Value, ParseScalarResult, ParseScalarValue};
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use date::Date;
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#[juniper::graphql_scalar(description = " Date" )]
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impl< S> GraphQLScalar for Date
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where
S: ScalarValue
{
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// Define how to convert your custom scalar into a primitive type.
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fn resolve(& self) -> Value {
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Value::scalar(self.to_string())
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}
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// Define how to parse a primitive type into your custom scalar.
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// NOTE: The error type should implement `IntoFieldError< S> `.
fn from_input_value(v: & InputValue) -> Result< Date, String> {
v.as_string_value()
.ok_or_else(|| format!(" Expected `String`, found: {}" , v))
.and_then(|s| s.parse().map_err(|e| format!(" Failed to parse `Date`: {}" , e)))
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}
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// Define how to parse a string value.
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fn from_str< 'a> (value: ScalarToken< 'a> ) -> ParseScalarResult< 'a, S> {
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< String as ParseScalarValue< S> > ::from_str(value)
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}
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}
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#
# fn main() {}
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