Trait diesel::deserialize::Queryable
source · [−]pub trait Queryable<ST, DB> where
DB: Backend, {
type Row: FromSqlRow<ST, DB>;
fn build(row: Self::Row) -> Self;
}Expand description
Trait indicating that a record can be queried from the database.
Types which implement Queryable represent the result of a SQL query. This
does not necessarily mean they represent a single database table.
Diesel represents the return type of a query as a tuple. The purpose of this trait is to convert from a tuple of Rust values that have been deserialized into your struct.
Deriving
This trait can be derived automatically using #[derive(Queryable)]. This
trait can only be derived for structs, not enums.
When this trait is derived, it will assume that the order of fields on your
struct match the order of the fields in the query. This means that field
order is significant if you are using #[derive(Queryable)]. Field name has
no effect.
To provide custom deserialization behavior for a field, you can use
#[diesel(deserialize_as = "Type")]. If this attribute is present, Diesel
will deserialize into that type, rather than the type on your struct and
call .into to convert it. This can be used to add custom behavior for a
single field, or use types that are otherwise unsupported by Diesel.
Examples
If we just want to map a query to our struct, we can use derive.
#[derive(Queryable, PartialEq, Debug)]
struct User {
id: i32,
name: String,
}
let first_user = users.first(&connection)?;
let expected = User { id: 1, name: "Sean".into() };
assert_eq!(expected, first_user);If we want to do additional work during deserialization, we can use
deserialize_as to use a different implementation.
struct LowercaseString(String);
impl Into<String> for LowercaseString {
fn into(self) -> String {
self.0
}
}
impl<DB, ST> Queryable<ST, DB> for LowercaseString
where
DB: Backend,
String: Queryable<ST, DB>,
{
type Row = <String as Queryable<ST, DB>>::Row;
fn build(row: Self::Row) -> Self {
LowercaseString(String::build(row).to_lowercase())
}
}
#[derive(Queryable, PartialEq, Debug)]
struct User {
id: i32,
#[diesel(deserialize_as = "LowercaseString")]
name: String,
}
let first_user = users.first(&connection)?;
let expected = User { id: 1, name: "sean".into() };
assert_eq!(expected, first_user);Alternatively, we can implement the trait for our struct manually.
use schema::users;
use diesel::deserialize::Queryable;
type DB = diesel::sqlite::Sqlite;
#[derive(PartialEq, Debug)]
struct User {
id: i32,
name: String,
}
impl Queryable<users::SqlType, DB> for User {
type Row = (i32, String);
fn build(row: Self::Row) -> Self {
User {
id: row.0,
name: row.1.to_lowercase(),
}
}
}
let first_user = users.first(&connection)?;
let expected = User { id: 1, name: "sean".into() };
assert_eq!(expected, first_user);Associated Types
type Row: FromSqlRow<ST, DB>
type Row: FromSqlRow<ST, DB>
The Rust type you’d like to map from.
This is typically a tuple of all of your struct’s fields.