pub trait QueryId {
type QueryId: Any;
const HAS_STATIC_QUERY_ID: bool = true;
// Provided method
fn query_id() -> Option<TypeId> { ... }
}Expand description
Uniquely identifies queries by their type for the purpose of prepared statement caching.
All types which implement QueryFragment should also implement this trait
(It is not an actual supertrait of QueryFragment for boxing purposes).
See the documentation of the QueryId type and HAS_STATIC_QUERY_ID
for more details.
§Deriving
This trait can be automatically derived by Diesel. For example, given this struct:
#[derive(QueryId)]
pub struct And<Left, Right> {
left: Left,
right: Right,
}the following implementation will be generated
ⓘ
impl<Left, Right> QueryId for And<Left, Right>
where
Left: QueryId,
Right: QueryId,
{
type QueryId = And<Left::QueryId, Right::QueryId>;
const HAS_STATIC_QUERY_ID: bool = Left::HAS_STATIC_QUERY_ID && Right::HAS_STATIC_QUERY_ID;
}If the SQL generated by a struct is not uniquely identifiable by its type,
meaning that HAS_STATIC_QUERY_ID should always be false,
you should not derive this trait.
In that case you should manually implement it instead.
Provided Associated Constants§
Sourceconst HAS_STATIC_QUERY_ID: bool = true
const HAS_STATIC_QUERY_ID: bool = true
Can the SQL generated by Self be uniquely identified by its type?
Typically this question can be answered by looking at whether
unsafe_to_cache_prepared is called in your implementation of
QueryFragment::walk_ast. In Diesel itself, the only type which has
false here, but is potentially safe to store in the prepared statement
cache is a boxed query.
Required Associated Types§
Sourcetype QueryId: Any
type QueryId: Any
A type which uniquely represents Self in a SQL query.
Typically this will be a re-construction of Self using the QueryId
type of each of your type parameters. For example, the type And<Left, Right> would have type QueryId = And<Left::QueryId, Right::QueryId>.
The exception to this is when one of your type parameters does not
affect whether the same prepared statement can be used or not. For
example, a bind parameter is represented as Bound<SqlType, RustType>.
The actual Rust type we are serializing does not matter for the purposes
of prepared statement reuse, but a query which has identical SQL but
different types for its bind parameters requires a new prepared
statement. For this reason, Bound would have type QueryId = Bound<SqlType::QueryId, ()>.
If HAS_STATIC_QUERY_ID is false, you can put any type here
(typically ()).
Provided Methods§
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.