Trait QueryKindTrait 

pub trait QueryKindTrait<Value, OnEmpty> {
    type Query: FullCodec + 'static;

    const METADATA: StorageEntryModifierIR;

    // Required methods
    fn from_optional_value_to_query(v: Option<Value>) -> Self::Query;
    fn from_query_to_optional_value(v: Self::Query) -> Option<Value>;
}

Trait implementing how the storage optional value is converted into the queried type.

It is implemented most notable by:

• OptionQuery which converts an optional value to an optional value, used when querying storage returns an optional value.
• ResultQuery which converts an optional value to a result value, used when querying storage returns a result value.
• ValueQuery which converts an optional value to a value, used when querying storage returns a value.

Example

#[test]
pub fn value_query_examples() {
	/// Custom default impl to be used with `ValueQuery`.
	struct UniverseSecret;
	impl Get<u32> for UniverseSecret {
		fn get() -> u32 {
			42
		}
	}

	/// Custom default impl to be used with `ResultQuery`.
	struct GetDefaultForResult;
	impl Get<Result<u32, ()>> for GetDefaultForResult {
		fn get() -> Result<u32, ()> {
			Err(())
		}
	}

	type A = StorageValue<Prefix, u32, ValueQuery>;
	type B = StorageValue<Prefix, u32, OptionQuery>;
	type C = StorageValue<Prefix, u32, ResultQuery<()>, GetDefaultForResult>;
	type D = StorageValue<Prefix, u32, ValueQuery, UniverseSecret>;

	TestExternalities::default().execute_with(|| {
		// normal value query returns default
		assert_eq!(A::get(), 0);

		// option query returns none
		assert_eq!(B::get(), None);

		// result query returns error
		assert_eq!(C::get(), Err(()));

		// value query with custom on empty returns 42
		assert_eq!(D::get(), 42);
	});
}

Required Associated Constants

const METADATA: StorageEntryModifierIR

Metadata for the storage kind.

Required Associated Types

type Query: FullCodec + 'static

Type returned on query

Required Methods

fn from_optional_value_to_query(v: Option<Value>) -> Self::Query

Convert an optional value (i.e. some if trie contains the value or none otherwise) to the query.

fn from_query_to_optional_value(v: Self::Query) -> Option<Value>

Convert a query to an optional value.

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors

impl<Value> QueryKindTrait<Value, GetDefault> for OptionQuery

where Value: FullCodec + 'static,

const METADATA: StorageEntryModifierIR = StorageEntryModifierIR::Optional

type Query = Option<Value>

impl<Value, Error, OnEmpty> QueryKindTrait<Value, OnEmpty> for ResultQuery<Error>

where Value: FullCodec + 'static, Error: FullCodec + 'static, OnEmpty: Get<Result<Value, Error>>,

const METADATA: StorageEntryModifierIR = StorageEntryModifierIR::Optional

type Query = Result<Value, Error>

impl<Value, OnEmpty> QueryKindTrait<Value, OnEmpty> for ValueQuery

where Value: FullCodec + 'static, OnEmpty: Get<Value>,

const METADATA: StorageEntryModifierIR = StorageEntryModifierIR::Default

type Query = Value