Trait diesel::prelude::Queryable

source ·

pub trait Queryable<ST, DB>: Sizedwhere
    DB: Backend,{
    type Row: FromStaticSqlRow<ST, DB>;

    // Required method
    fn build(row: Self::Row) -> Result<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.

This trait can be derived

How to resolve compiler errors while loading data from the database

In case you getting uncomprehensable compiler errors while loading data from the database you might want to consider using #[derive(Selectable)] + #[diesel(check_for_backend(YourBackendType))] to check for mismatching fields at compile time. This drastically improves the quality of the generated error messages by pointing to concrete mismatches at field level. You need to specify the concrete database backend this specific struct is indented to be used with, as otherwise rustc cannot correctly identify the required deserialization implementation.

Interaction with `NULL`/`Option`

Nullable types can be queried into Option. This is valid for single fields, tuples, and structures with Queryable.

With tuples and structs, the process for deserializing an Option<(A,B,C)> is to attempt to deserialize A, B and C, and if either of these return an UnexpectedNullError, the Option will be deserialized as None. If all succeed, the Option will be deserialized as Some((a,b,c)).

Examples

Simple mapping from query to struct

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.order_by(id).first(connection)?;
let expected = User { id: 1, name: "Sean".into() };
assert_eq!(expected, first_user);

Interaction with `NULL`/`Option`

Single field

table! {
    animals {
        id -> Integer,
        species -> VarChar,
        legs -> Integer,
        name -> Nullable<VarChar>,
    }
}
#[derive(Queryable, PartialEq, Debug)]
struct Animal {
    id: i32,
    name: Option<String>,
}

let all_animals = animals.select((id, name)).order_by(id).load(connection)?;
let expected = vec![Animal { id: 1, name: Some("Jack".to_owned()) }, Animal { id: 2, name: None }];
assert_eq!(expected, all_animals);

Multiple fields

#[derive(Queryable, PartialEq, Debug)]
struct UserWithPost {
    id: i32,
    post: Option<Post>,
}
#[derive(Queryable, PartialEq, Debug)]
struct Post {
    id: i32,
    title: String,
}

let all_posts = users::table
    .left_join(posts::table)
    .select((
        users::id,
        (posts::id, posts::title).nullable()
    ))
    .order_by((users::id, posts::id))
    .load(connection)?;
let expected = vec![
    UserWithPost { id: 1, post: Some(Post { id: 1, title: "My first post".to_owned() }) },
    UserWithPost { id: 1, post: Some(Post { id: 2, title: "About Rust".to_owned() }) },
    UserWithPost { id: 2, post: Some(Post { id: 3, title: "My first post too".to_owned() }) },
    UserWithPost { id: 3, post: None },
];
assert_eq!(expected, all_posts);

`deserialize_as` attribute

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> Queryable<Text, DB> for LowercaseString
where
    DB: Backend,
    String: FromSql<Text, DB>,
{
    type Row = String;

    fn build(s: String) -> deserialize::Result<Self> {
        Ok(LowercaseString(s.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);

Manual implementation

Alternatively, we can implement the trait for our struct manually.

use schema::users;
use diesel::deserialize::{self, 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) -> deserialize::Result<Self> {
        Ok(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);

Required Associated Types§

source

type Row: FromStaticSqlRow<ST, DB>

The Rust type you’d like to map from.

This is typically a tuple of all of your struct’s fields.

Required Methods§

source

fn build(row: Self::Row) -> Result<Self>

Construct an instance of this type

Trait diesel::prelude::Queryable

Required Associated Types§

type Row: FromStaticSqlRow<ST, DB>

Required Methods§

fn build(row: Self::Row) -> Result<Self>

Implementations on Foreign Types§

impl<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8, ST9> Queryable<Record<(ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8, ST9)>, Pg> for (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9)where Self: FromSql<Record<(ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8, ST9)>, Pg>,

type Row = (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9)

fn build(row: Self::Row) -> Result<Self>

type Row = (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19, T20, T21, T22, T23, T24, T25, T26)

fn build(row: Self::Row) -> Result<Self>

type Row = Option<__T>

fn build(row: Self::Row) -> Result<Self>

type Row = (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19, T20, T21, T22, T23, T24, T25, T26, T27, T28, T29, T30, T31)

fn build(row: Self::Row) -> Result<Self>

impl<__T, __DB, ST0, ST1, ST2, ST3> Queryable<Nullable<(ST0, ST1, ST2, ST3)>, __DB> for Option<__T>where __DB: Backend, Self: FromStaticSqlRow<Nullable<(ST0, ST1, ST2, ST3)>, __DB>, (ST0, ST1, ST2, ST3): SqlType,

type Row = Option<__T>

fn build(row: Self::Row) -> Result<Self>

type Row = (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19)

fn build(row: Self::Row) -> Result<Self>

impl Queryable<Text, Pg> for *const str

type Row = *const str

fn build(row: Self::Row) -> Result<Self>

type Row = Option<__T>

fn build(row: Self::Row) -> Result<Self>

impl<__ST, __DB> Queryable<__ST, __DB> for NaiveTimewhere __DB: Backend, __ST: SingleValue, Self: FromSql<__ST, __DB>,

type Row = NaiveTime

fn build(row: Self::Row) -> Result<Self>

impl<T0, T1, T2, T3, T4, T5, T6, T7, T8, __DB, ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8> Queryable<(ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8), __DB> for (T0, T1, T2, T3, T4, T5, T6, T7, T8)where __DB: Backend, Self: FromStaticSqlRow<(ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8), __DB>,

type Row = (T0, T1, T2, T3, T4, T5, T6, T7, T8)

fn build(row: Self::Row) -> Result<Self>

type Row = Option<__T>

fn build(row: Self::Row) -> Result<Self>

type Row = (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19, T20)

fn build(row: Self::Row) -> Result<Self>

impl<T0, T1, T2, T3, T4, T5, ST0, ST1, ST2, ST3, ST4, ST5> Queryable<Record<(ST0, ST1, ST2, ST3, ST4, ST5)>, Pg> for (T0, T1, T2, T3, T4, T5)where Self: FromSql<Record<(ST0, ST1, ST2, ST3, ST4, ST5)>, Pg>,

type Row = (T0, T1, T2, T3, T4, T5)

fn build(row: Self::Row) -> Result<Self>

type Row = Option<__T>

fn build(row: Self::Row) -> Result<Self>

type Row = (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15)

fn build(row: Self::Row) -> Result<Self>

impl<__ST, __DB> Queryable<__ST, __DB> for NaiveDatewhere __DB: Backend, __ST: SingleValue, Self: FromSql<__ST, __DB>,

type Row = NaiveDate

fn build(row: Self::Row) -> Result<Self>

impl<ST, T, DB> Queryable<ST, DB> for Option<T>where ST: SingleValue<IsNull = IsNullable>, DB: Backend, Self: FromSql<ST, DB>,

type Row = Option<T>

fn build(row: Self::Row) -> Result<Self>

type Row = (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19, T20, T21)

fn build(row: Self::Row) -> Result<Self>

type Row = Option<__T>

fn build(row: Self::Row) -> Result<Self>

type Row = (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19, T20, T21, T22, T23, T24, T25, T26, T27, T28)

fn build(row: Self::Row) -> Result<Self>

type Row = (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19, T20, T21, T22, T23, T24, T25, T26, T27, T28, T29, T30)

fn build(row: Self::Row) -> Result<Self>

type Row = Option<__T>

fn build(row: Self::Row) -> Result<Self>

type Row = (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19, T20, T21, T22, T23, T24, T25)

fn build(row: Self::Row) -> Result<Self>

type Row = Option<__T>

fn build(row: Self::Row) -> Result<Self>

impl<__ST, __DB> Queryable<__ST, __DB> for IpNetworkwhere __DB: Backend, __ST: SingleValue, Self: FromSql<__ST, __DB>,

type Row = IpNetwork

fn build(row: Self::Row) -> Result<Self>

impl<T0, T1, __DB, ST0, ST1> Queryable<(ST0, ST1), __DB> for (T0, T1)where __DB: Backend, Self: FromStaticSqlRow<(ST0, ST1), __DB>,

type Row = (T0, T1)

fn build(row: Self::Row) -> Result<Self>

type Row = (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12)

fn build(row: Self::Row) -> Result<Self>

impl<__ST, __DB> Queryable<__ST, __DB> for NaiveDateTimewhere __DB: Backend, __ST: SingleValue, Self: FromSql<__ST, __DB>,

type Row = NaiveDateTime

fn build(row: Self::Row) -> Result<Self>

type Row = (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17)

fn build(row: Self::Row) -> Result<Self>

type Row = Option<__T>

fn build(row: Self::Row) -> Result<Self>

type Row = (T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19)

fn build(row: Self::Row) -> Result<Self>

type Row = Option<__T>

impl<T, DB, ST0, ST1, ST2, ST3> Queryable<Nullable<(ST0, ST1, ST2, ST3)>, DB> for Option<T>where DB: Backend, Self: FromStaticSqlRow<Nullable<(ST0, ST1, ST2, ST3)>, DB>, (ST0, ST1, ST2, ST3): SqlType,

impl<ST, DB> Queryable<ST, DB> for NaiveTimewhere DB: Backend, ST: SingleValue, Self: FromSql<ST, DB>,

impl<T0, T1, T2, T3, T4, T5, T6, T7, T8, DB, ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8> Queryable<(ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8), DB> for (T0, T1, T2, T3, T4, T5, T6, T7, T8)where DB: Backend, Self: FromStaticSqlRow<(ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8), DB>,

impl<ST, DB> Queryable<ST, DB> for NaiveDatewhere DB: Backend, ST: SingleValue, Self: FromSql<ST, DB>,

impl<ST, DB> Queryable<ST, DB> for IpNetworkwhere DB: Backend, ST: SingleValue, Self: FromSql<ST, DB>,

impl<T0, T1, DB, ST0, ST1> Queryable<(ST0, ST1), DB> for (T0, T1)where DB: Backend, Self: FromStaticSqlRow<(ST0, ST1), DB>,

impl<ST, DB> Queryable<ST, DB> for NaiveDateTimewhere DB: Backend, ST: SingleValue, Self: FromSql<ST, DB>,

impl<T0, T1, T2, T3, DB, ST0, ST1, ST2, ST3> Queryable<(ST0, ST1, ST2, ST3), DB> for (T0, T1, T2, T3)where DB: Backend, Self: FromStaticSqlRow<(ST0, ST1, ST2, ST3), DB>,

impl<ST, DB> Queryable<ST, DB> for f64where DB: Backend, ST: SingleValue, Self: FromSql<ST, DB>,

impl<ST, DB> Queryable<ST, DB> for i8where DB: Backend, ST: SingleValue, Self: FromSql<ST, DB>,

impl<ST, DB> Queryable<ST, DB> for Stringwhere DB: Backend, ST: SingleValue, Self: FromSql<ST, DB>,