pub trait QueryDsl: Sized {
Show 26 methods
// Provided methods
fn distinct(self) -> Distinct<Self>
where Self: DistinctDsl { ... }
fn distinct_on<Expr>(self, expr: Expr) -> DistinctOn<Self, Expr>
where Self: DistinctOnDsl<Expr> { ... }
fn select<Selection>(self, selection: Selection) -> Select<Self, Selection>
where Selection: Expression,
Self: SelectDsl<Selection> { ... }
fn count(self) -> Select<Self, CountStar>
where Self: SelectDsl<CountStar> { ... }
fn inner_join<Rhs>(self, rhs: Rhs) -> InnerJoin<Self, Rhs>
where Self: JoinWithImplicitOnClause<Rhs, Inner> { ... }
fn left_outer_join<Rhs>(self, rhs: Rhs) -> LeftJoin<Self, Rhs>
where Self: JoinWithImplicitOnClause<Rhs, LeftOuter> { ... }
fn left_join<Rhs>(self, rhs: Rhs) -> LeftJoin<Self, Rhs>
where Self: JoinWithImplicitOnClause<Rhs, LeftOuter> { ... }
fn filter<Predicate>(self, predicate: Predicate) -> Filter<Self, Predicate>
where Self: FilterDsl<Predicate> { ... }
fn or_filter<Predicate>(
self,
predicate: Predicate,
) -> OrFilter<Self, Predicate>
where Self: OrFilterDsl<Predicate> { ... }
fn find<PK>(self, id: PK) -> Find<Self, PK>
where Self: FindDsl<PK> { ... }
fn order<Expr>(self, expr: Expr) -> Order<Self, Expr>
where Expr: Expression,
Self: OrderDsl<Expr> { ... }
fn order_by<Expr>(self, expr: Expr) -> Order<Self, Expr>
where Expr: Expression,
Self: OrderDsl<Expr> { ... }
fn then_order_by<Order>(self, order: Order) -> ThenOrderBy<Self, Order>
where Self: ThenOrderDsl<Order> { ... }
fn limit(self, limit: i64) -> Limit<Self>
where Self: LimitDsl { ... }
fn offset(self, offset: i64) -> Offset<Self>
where Self: OffsetDsl { ... }
fn group_by<GB>(self, group_by: GB) -> GroupBy<Self, GB>
where GB: Expression,
Self: GroupByDsl<GB> { ... }
fn having<Predicate>(self, predicate: Predicate) -> Having<Self, Predicate>
where Self: HavingDsl<Predicate> { ... }
fn for_update(self) -> ForUpdate<Self>
where Self: LockingDsl<ForUpdate> { ... }
fn for_no_key_update(self) -> ForNoKeyUpdate<Self>
where Self: LockingDsl<ForNoKeyUpdate> { ... }
fn for_share(self) -> ForShare<Self>
where Self: LockingDsl<ForShare> { ... }
fn for_key_share(self) -> ForKeyShare<Self>
where Self: LockingDsl<ForKeyShare> { ... }
fn skip_locked(self) -> SkipLocked<Self>
where Self: ModifyLockDsl<SkipLocked> { ... }
fn no_wait(self) -> NoWait<Self>
where Self: ModifyLockDsl<NoWait> { ... }
fn into_boxed<'a, DB>(self) -> IntoBoxed<'a, Self, DB>
where DB: Backend,
Self: BoxedDsl<'a, DB> { ... }
fn single_value(self) -> SingleValue<Self>
where Self: SingleValueDsl { ... }
fn nullable(self) -> NullableSelect<Self>
where Self: SelectNullableDsl { ... }
}
Expand description
Methods used to construct select statements.
Provided Methods§
sourcefn distinct(self) -> Distinct<Self>where
Self: DistinctDsl,
fn distinct(self) -> Distinct<Self>where
Self: DistinctDsl,
Adds the DISTINCT
keyword to a query.
This method will override any previous distinct clause that was present.
For example, on PostgreSQL, foo.distinct_on(bar).distinct()
will
create the same query as foo.distinct()
.
§Example
diesel::insert_into(users)
.values(&vec![name.eq("Sean"); 3])
.execute(connection)?;
let names = users.select(name).load::<String>(connection)?;
let distinct_names = users.select(name).distinct().load::<String>(connection)?;
assert_eq!(vec!["Sean"; 3], names);
assert_eq!(vec!["Sean"; 1], distinct_names);
sourcefn distinct_on<Expr>(self, expr: Expr) -> DistinctOn<Self, Expr>where
Self: DistinctOnDsl<Expr>,
fn distinct_on<Expr>(self, expr: Expr) -> DistinctOn<Self, Expr>where
Self: DistinctOnDsl<Expr>,
Adds the DISTINCT ON
clause to a query.
§Example
diesel::insert_into(animals)
.values(&vec![
(species.eq("dog"), name.eq(Some("Jack")), legs.eq(4)),
(species.eq("dog"), name.eq(None), legs.eq(4)),
(species.eq("spider"), name.eq(None), legs.eq(8)),
])
.execute(connection)
.unwrap();
let all_animals = animals.select((species, name, legs)).load(connection);
let distinct_animals = animals
.select((species, name, legs))
.order_by((species, legs))
.distinct_on(species)
.load(connection);
assert_eq!(Ok(vec![Animal::new("dog", Some("Jack"), 4),
Animal::new("dog", None, 4),
Animal::new("spider", None, 8)]), all_animals);
assert_eq!(Ok(vec![Animal::new("dog", Some("Jack"), 4),
Animal::new("spider", None, 8)]), distinct_animals);
sourcefn select<Selection>(self, selection: Selection) -> Select<Self, Selection>where
Selection: Expression,
Self: SelectDsl<Selection>,
fn select<Selection>(self, selection: Selection) -> Select<Self, Selection>where
Selection: Expression,
Self: SelectDsl<Selection>,
Adds a SELECT
clause to the query.
If there was already a select clause present, it will be overridden.
For example, foo.select(bar).select(baz)
will produce the same
query as foo.select(baz)
.
By default, the select clause will be roughly equivalent to SELECT *
(however, Diesel will list all columns to ensure that they are in the
order we expect).
select
has slightly stricter bounds on its arguments than other
methods. In particular, when used with a left outer join, .nullable
must be called on columns that come from the right side of a join. It
can be called on the column itself, or on an expression containing that
column. title.nullable()
, lower(title).nullable()
, and (id, title).nullable()
would all be valid.
§Examples
// By default, all columns will be selected
let all_users = users.load::<(i32, String)>(connection)?;
assert_eq!(vec![(1, String::from("Sean")), (2, String::from("Tess"))], all_users);
let all_names = users.select(name).load::<String>(connection)?;
assert_eq!(vec!["Sean", "Tess"], all_names);
§When used with a left join
let join = users::table.left_join(posts::table);
// By default, all columns from both tables are selected.
// If no explicit select clause is used this means that the result
// type of this query must contain all fields from the original schema in order.
let all_data = join.load::<(User, Option<Post>)>(connection)?;
let expected_data = vec![
(User::new(1, "Sean"), Some(Post::new(post_id, 1, "Sean's Post"))),
(User::new(2, "Tess"), None),
];
assert_eq!(expected_data, all_data);
// Since `posts` is on the right side of a left join, `.nullable` is
// needed.
let names_and_titles = join.select((users::name, posts::title.nullable()))
.load::<(String, Option<String>)>(connection)?;
let expected_data = vec![
(String::from("Sean"), Some(String::from("Sean's Post"))),
(String::from("Tess"), None),
];
assert_eq!(expected_data, names_and_titles);
sourcefn count(self) -> Select<Self, CountStar>
fn count(self) -> Select<Self, CountStar>
Get the count of a query. This is equivalent to .select(count_star())
§Example
let count = users.count().get_result(connection);
assert_eq!(Ok(2), count);
sourcefn inner_join<Rhs>(self, rhs: Rhs) -> InnerJoin<Self, Rhs>where
Self: JoinWithImplicitOnClause<Rhs, Inner>,
fn inner_join<Rhs>(self, rhs: Rhs) -> InnerJoin<Self, Rhs>where
Self: JoinWithImplicitOnClause<Rhs, Inner>,
Join two tables using a SQL INNER JOIN
.
If you have invoked joinable!
for the two tables, you can pass that
table directly. Otherwise you will need to use .on
to specify the ON
clause.
You can join to as many tables as you’d like in a query, with the
restriction that no table can appear in the query more than once. For
tables that appear more than once in a single query the usage of alias!
is required.
You will also need to call allow_tables_to_appear_in_same_query!
.
If you are using diesel print-schema
, this will
have been generated for you.
See the documentation for allow_tables_to_appear_in_same_query!
for
details.
Diesel expects multi-table joins to be semantically grouped based on the
relationships. For example, users.inner_join(posts.inner_join(comments))
is not the same as users.inner_join(posts).inner_join(comments)
. The first
would deserialize into (User, (Post, Comment))
and generate the following
SQL:
SELECT * FROM users
INNER JOIN (
posts
INNER JOIN comments ON comments.post_id = posts.id
) ON posts.user_id = users.id
While the second query would deserialize into (User, Post, Comment)
and
generate the following SQL:
SELECT * FROM users
INNER JOIN posts ON posts.user_id = users.id
INNER JOIN comments ON comments.user_id = users.id
The exact generated SQL may change in future diesel version as long as the
generated query continues to produce same results. The currently generated
SQL is referred as “explicit join”
by the PostgreSQL documentation and may have implications on the chosen query plan
for large numbers of joins in the same query. Checkout the documentation of the
join_collapse_limit
paramater
to control this behaviour.
§Examples
§With implicit ON
clause
joinable!(posts -> users (user_id));
allow_tables_to_appear_in_same_query!(users, posts);
let data = users.inner_join(posts)
.select((name, title))
.load(connection);
let expected_data = vec![
(String::from("Sean"), String::from("My first post")),
(String::from("Sean"), String::from("About Rust")),
(String::from("Tess"), String::from("My first post too")),
];
assert_eq!(Ok(expected_data), data);
§With explicit ON
clause
allow_tables_to_appear_in_same_query!(users, posts);
diesel::insert_into(posts)
.values(&vec![
(user_id.eq(1), title.eq("Sean's post")),
(user_id.eq(2), title.eq("Sean is a jerk")),
])
.execute(connection)
.unwrap();
let data = users
.inner_join(posts.on(title.like(name.concat("%"))))
.select((name, title))
.load(connection);
let expected_data = vec![
(String::from("Sean"), String::from("Sean's post")),
(String::from("Sean"), String::from("Sean is a jerk")),
];
assert_eq!(Ok(expected_data), data);
§With explicit ON
clause (struct)
allow_tables_to_appear_in_same_query!(users, posts);
#[derive(Debug, PartialEq, Queryable)]
struct User {
id: i32,
name: String,
}
#[derive(Debug, PartialEq, Queryable)]
struct Post {
id: i32,
user_id: i32,
title: String,
}
diesel::insert_into(posts)
.values(&vec![
(user_id.eq(1), title.eq("Sean's post")),
(user_id.eq(2), title.eq("Sean is a jerk")),
])
.execute(connection)
.unwrap();
// By default, all columns from both tables are selected.
// If no explicit select clause is used this means that the
// result type of this query must contain all fields from the
// original schema in order.
let data = users
.inner_join(posts.on(title.like(name.concat("%"))))
.load::<(User, Post)>(connection); // type could be elided
let expected_data = vec![
(
User { id: 1, name: String::from("Sean") },
Post { id: 4, user_id: 1, title: String::from("Sean's post") },
),
(
User { id: 1, name: String::from("Sean") },
Post { id: 5, user_id: 2, title: String::from("Sean is a jerk") },
),
];
assert_eq!(Ok(expected_data), data);
sourcefn left_outer_join<Rhs>(self, rhs: Rhs) -> LeftJoin<Self, Rhs>where
Self: JoinWithImplicitOnClause<Rhs, LeftOuter>,
fn left_outer_join<Rhs>(self, rhs: Rhs) -> LeftJoin<Self, Rhs>where
Self: JoinWithImplicitOnClause<Rhs, LeftOuter>,
Join two tables using a SQL LEFT OUTER JOIN
.
Behaves similarly to inner_join
, but will produce a left join
instead. See inner_join
for usage examples.
Columns in the right hand table will become Nullable
which means
you must call nullable()
on the corresponding fields in the select
clause:
§Selecting after a left join
let join = users::table.left_join(posts::table);
// Since `posts` is on the right side of a left join, `.nullable` is
// needed.
let names_and_titles = join.select((users::name, posts::title.nullable()))
.load::<(String, Option<String>)>(connection)?;
let expected_data = vec![
(String::from("Sean"), Some(String::from("Sean's Post"))),
(String::from("Tess"), None),
];
assert_eq!(expected_data, names_and_titles);
sourcefn left_join<Rhs>(self, rhs: Rhs) -> LeftJoin<Self, Rhs>where
Self: JoinWithImplicitOnClause<Rhs, LeftOuter>,
fn left_join<Rhs>(self, rhs: Rhs) -> LeftJoin<Self, Rhs>where
Self: JoinWithImplicitOnClause<Rhs, LeftOuter>,
Alias for left_outer_join
.
sourcefn filter<Predicate>(self, predicate: Predicate) -> Filter<Self, Predicate>where
Self: FilterDsl<Predicate>,
fn filter<Predicate>(self, predicate: Predicate) -> Filter<Self, Predicate>where
Self: FilterDsl<Predicate>,
Adds to the WHERE
clause of a query.
If there is already a WHERE
clause, the result will be old AND new
.
§Example:
let seans_id = users.filter(name.eq("Sean")).select(id)
.first(connection);
assert_eq!(Ok(1), seans_id);
let tess_id = users.filter(name.eq("Tess")).select(id)
.first(connection);
assert_eq!(Ok(2), tess_id);
sourcefn or_filter<Predicate>(self, predicate: Predicate) -> OrFilter<Self, Predicate>where
Self: OrFilterDsl<Predicate>,
fn or_filter<Predicate>(self, predicate: Predicate) -> OrFilter<Self, Predicate>where
Self: OrFilterDsl<Predicate>,
Adds to the WHERE
clause of a query using OR
If there is already a WHERE
clause, the result will be (old OR new)
.
Calling foo.filter(bar).or_filter(baz)
is identical to foo.filter(bar.or(baz))
.
However, the second form is much harder to do dynamically.
§Example:
diesel::insert_into(animals)
.values(&vec![
(species.eq("cat"), legs.eq(4), name.eq("Sinatra")),
(species.eq("dog"), legs.eq(3), name.eq("Fido")),
(species.eq("spider"), legs.eq(8), name.eq("Charlotte")),
])
.execute(connection)?;
let good_animals = animals
.filter(name.eq("Fido"))
.or_filter(legs.eq(4))
.select(name)
.get_results::<Option<String>>(connection)?;
let expected = vec![
Some(String::from("Sinatra")),
Some(String::from("Fido")),
];
assert_eq!(expected, good_animals);
sourcefn find<PK>(self, id: PK) -> Find<Self, PK>where
Self: FindDsl<PK>,
fn find<PK>(self, id: PK) -> Find<Self, PK>where
Self: FindDsl<PK>,
Attempts to find a single record from the given table by primary key.
§Example
let sean = (1, "Sean".to_string());
let tess = (2, "Tess".to_string());
assert_eq!(Ok(sean), users.find(1).first(connection));
assert_eq!(Ok(tess), users.find(2).first(connection));
assert_eq!(Err::<(i32, String), _>(NotFound), users.find(3).first(connection));
sourcefn order<Expr>(self, expr: Expr) -> Order<Self, Expr>where
Expr: Expression,
Self: OrderDsl<Expr>,
fn order<Expr>(self, expr: Expr) -> Order<Self, Expr>where
Expr: Expression,
Self: OrderDsl<Expr>,
Sets the order clause of a query.
If there was already an order clause, it will be overridden. See
also:
.desc()
and
.asc()
Ordering by multiple columns can be achieved by passing a tuple of those
columns.
To construct an order clause of an unknown number of columns,
see QueryDsl::then_order_by
§Examples
diesel::insert_into(users)
.values(&vec![
name.eq("Saul"),
name.eq("Steve"),
name.eq("Stan"),
])
.execute(connection)?;
let ordered_names = users.select(name)
.order(name.desc())
.load::<String>(connection)?;
assert_eq!(vec!["Steve", "Stan", "Saul"], ordered_names);
diesel::insert_into(users).values(name.eq("Stan")).execute(connection)?;
let data = users.select((name, id))
.order((name.asc(), id.desc()))
.load(connection)?;
let expected_data = vec![
(String::from("Saul"), 3),
(String::from("Stan"), 6),
(String::from("Stan"), 5),
(String::from("Steve"), 4),
];
assert_eq!(expected_data, data);
sourcefn order_by<Expr>(self, expr: Expr) -> Order<Self, Expr>where
Expr: Expression,
Self: OrderDsl<Expr>,
fn order_by<Expr>(self, expr: Expr) -> Order<Self, Expr>where
Expr: Expression,
Self: OrderDsl<Expr>,
Alias for order
sourcefn then_order_by<Order>(self, order: Order) -> ThenOrderBy<Self, Order>where
Self: ThenOrderDsl<Order>,
fn then_order_by<Order>(self, order: Order) -> ThenOrderBy<Self, Order>where
Self: ThenOrderDsl<Order>,
Appends to the ORDER BY
clause of this SQL query.
Unlike .order
, this method will append rather than replace.
In other words,
.order_by(foo).order_by(bar)
is equivalent to .order_by(bar)
.
In contrast,
.order_by(foo).then_order_by(bar)
is equivalent to .order((foo, bar))
.
This method is only present on boxed queries.
§Examples
diesel::insert_into(users)
.values(&vec![
name.eq("Saul"),
name.eq("Steve"),
name.eq("Stan"),
name.eq("Stan"),
])
.execute(connection)?;
let data = users.select((name, id))
.order_by(name.asc())
.then_order_by(id.desc())
.load(connection)?;
let expected_data = vec![
(String::from("Saul"), 3),
(String::from("Stan"), 6),
(String::from("Stan"), 5),
(String::from("Steve"), 4),
];
assert_eq!(expected_data, data);
sourcefn limit(self, limit: i64) -> Limit<Self>where
Self: LimitDsl,
fn limit(self, limit: i64) -> Limit<Self>where
Self: LimitDsl,
Sets the limit clause of the query.
If there was already a limit clause, it will be overridden.
§Example
// Using a limit
let limited = users.select(name)
.order(id)
.limit(1)
.load::<String>(connection)?;
// Without a limit
let no_limit = users.select(name)
.order(id)
.load::<String>(connection)?;
assert_eq!(vec!["Sean"], limited);
assert_eq!(vec!["Sean", "Bastien", "Pascal"], no_limit);
sourcefn offset(self, offset: i64) -> Offset<Self>where
Self: OffsetDsl,
fn offset(self, offset: i64) -> Offset<Self>where
Self: OffsetDsl,
Sets the offset clause of the query.
If there was already a offset clause, it will be overridden.
§Example
// Using an offset
let offset = users.select(name)
.order(id)
.limit(2)
.offset(1)
.load::<String>(connection)?;
// No Offset
let no_offset = users.select(name)
.order(id)
.limit(2)
.load::<String>(connection)?;
assert_eq!(vec!["Bastien", "Pascal"], offset);
assert_eq!(vec!["Sean", "Bastien"], no_offset);
sourcefn group_by<GB>(self, group_by: GB) -> GroupBy<Self, GB>where
GB: Expression,
Self: GroupByDsl<GB>,
fn group_by<GB>(self, group_by: GB) -> GroupBy<Self, GB>where
GB: Expression,
Self: GroupByDsl<GB>,
Sets the group by
clause of a query.
Note: Queries having a group by
clause require a custom select clause.
Use QueryDsl::select()
to specify one.
If there was already a group by clause, it will be overridden. Ordering by multiple columns can be achieved by passing a tuple of those columns.
Diesel follows postgresql’s group by semantic, this means any column appearing in a group by clause is considered to be aggregated. If a primary key is part of the group by clause every column from the corresponding table is considerd to be aggregated. Select clauses cannot mix aggregated and non aggregated expressions.
For group by clauses containing columns from more than one table it
is required to call allow_columns_to_appear_in_same_group_by_clause!
§Examples
let data = users::table.inner_join(posts::table)
.group_by(users::id)
.select((users::name, count(posts::id)))
.load::<(String, i64)>(connection)?;
assert_eq!(vec![(String::from("Sean"), 2), (String::from("Tess"), 1)], data);
sourcefn having<Predicate>(self, predicate: Predicate) -> Having<Self, Predicate>where
Self: HavingDsl<Predicate>,
fn having<Predicate>(self, predicate: Predicate) -> Having<Self, Predicate>where
Self: HavingDsl<Predicate>,
Adds to the HAVING
clause of a query.
§Examples
let data = users::table.inner_join(posts::table)
.group_by(users::id)
.having(count(posts::id).gt(1))
.select((users::name, count(posts::id)))
.load::<(String, i64)>(connection)?;
assert_eq!(vec![(String::from("Sean"), 2)], data);
sourcefn for_update(self) -> ForUpdate<Self>where
Self: LockingDsl<ForUpdate>,
fn for_update(self) -> ForUpdate<Self>where
Self: LockingDsl<ForUpdate>,
Adds FOR UPDATE
to the end of the select statement.
This method is only available for MySQL and PostgreSQL. SQLite does not provide any form of row locking.
Additionally, .for_update
cannot be used on queries with a distinct
clause, group by clause, having clause, or any unions. Queries with
a FOR UPDATE
clause cannot be boxed.
§Example
// Executes `SELECT * FROM users FOR UPDATE`
let users_for_update = users::table.for_update().load(connection)?;
sourcefn for_no_key_update(self) -> ForNoKeyUpdate<Self>where
Self: LockingDsl<ForNoKeyUpdate>,
fn for_no_key_update(self) -> ForNoKeyUpdate<Self>where
Self: LockingDsl<ForNoKeyUpdate>,
Adds FOR NO KEY UPDATE
to the end of the select statement.
This method is only available for PostgreSQL. SQLite does not provide any form of row locking, and MySQL does not support anything finer than row-level locking.
Additionally, .for_no_key_update
cannot be used on queries with a distinct
clause, group by clause, having clause, or any unions. Queries with
a FOR NO KEY UPDATE
clause cannot be boxed.
§Example
// Executes `SELECT * FROM users FOR NO KEY UPDATE`
let users_for_no_key_update = users::table.for_no_key_update().load(connection)?;
Adds FOR SHARE
to the end of the select statement.
This method is only available for MySQL and PostgreSQL. SQLite does not provide any form of row locking.
Additionally, .for_share
cannot be used on queries with a distinct
clause, group by clause, having clause, or any unions. Queries with
a FOR SHARE
clause cannot be boxed.
§Example
// Executes `SELECT * FROM users FOR SHARE`
let users_for_share = users::table.for_share().load(connection)?;
Adds FOR KEY SHARE
to the end of the select statement.
This method is only available for PostgreSQL. SQLite does not provide any form of row locking, and MySQL does not support anything finer than row-level locking.
Additionally, .for_key_share
cannot be used on queries with a distinct
clause, group by clause, having clause, or any unions. Queries with
a FOR KEY SHARE
clause cannot be boxed.
§Example
// Executes `SELECT * FROM users FOR KEY SHARE`
let users_for_key_share = users::table.for_key_share().load(connection)?;
sourcefn skip_locked(self) -> SkipLocked<Self>where
Self: ModifyLockDsl<SkipLocked>,
fn skip_locked(self) -> SkipLocked<Self>where
Self: ModifyLockDsl<SkipLocked>,
Adds SKIP LOCKED
to the end of a FOR UPDATE
clause.
This modifier is only supported in PostgreSQL 9.5+ and MySQL 8+.
§Example
// Executes `SELECT * FROM users FOR UPDATE SKIP LOCKED`
let user_skiped_locked = users::table.for_update().skip_locked().load(connection)?;
sourcefn no_wait(self) -> NoWait<Self>where
Self: ModifyLockDsl<NoWait>,
fn no_wait(self) -> NoWait<Self>where
Self: ModifyLockDsl<NoWait>,
Adds NOWAIT
to the end of a FOR UPDATE
clause.
This modifier is only supported in PostgreSQL 9.5+ and MySQL 8+.
§Example
// Executes `SELECT * FROM users FOR UPDATE NOWAIT`
let users_no_wait = users::table.for_update().no_wait().load(connection)?;
sourcefn into_boxed<'a, DB>(self) -> IntoBoxed<'a, Self, DB>
fn into_boxed<'a, DB>(self) -> IntoBoxed<'a, Self, DB>
Boxes the pieces of a query into a single type.
This is useful for cases where you want to conditionally modify a query, but need the type to remain the same. The backend must be specified as part of this. It is not possible to box a query and have it be useable on multiple backends.
A boxed query will incur a minor performance penalty, as the query builder can no longer be inlined by the compiler. For most applications this cost will be minimal.
§Example
let mut query = users::table.into_boxed();
if let Some(name) = params.get("name") {
query = query.filter(users::name.eq(name));
}
let users = query.load(connection);
Diesel queries also have a similar problem to Iterator
, where
returning them from a function requires exposing the implementation of that
function. The helper_types
module exists to help with this,
but you might want to hide the return type or have it conditionally change.
Boxing can achieve both.
§Example
fn users_by_name(name: &str) -> users::BoxedQuery<DB> {
users::table.filter(users::name.eq(name)).into_boxed()
}
assert_eq!(Ok(1), users_by_name("Sean").select(users::id).first(connection));
assert_eq!(Ok(2), users_by_name("Tess").select(users::id).first(connection));
sourcefn single_value(self) -> SingleValue<Self>where
Self: SingleValueDsl,
fn single_value(self) -> SingleValue<Self>where
Self: SingleValueDsl,
Wraps this select statement in parenthesis, allowing it to be used as an expression.
SQL allows queries such as foo = (SELECT ...)
, as long as the
subselect returns only a single column, and 0 or 1 rows. This method
indicates that you expect the query to only return a single value (this
will be enforced by adding LIMIT 1
).
The SQL type of this will always be Nullable
, as the query returns
NULL
if the table is empty or it otherwise returns 0 rows.
§Example
insert_into(posts::table)
.values(posts::user_id.eq(1))
.execute(connection)?;
let last_post = posts::table
.order(posts::id.desc());
let most_recently_active_user = users.select(name)
.filter(id.nullable().eq(last_post.select(posts::user_id).single_value()))
.first::<String>(connection)?;
assert_eq!("Sean", most_recently_active_user);
sourcefn nullable(self) -> NullableSelect<Self>where
Self: SelectNullableDsl,
fn nullable(self) -> NullableSelect<Self>where
Self: SelectNullableDsl,
Coerce the SQL type of the select clause to it’s nullable equivalent.
This is use full for writing queries that contain subselects on non null fields comparing them to nullable fields.
table! {
users {
id -> Integer,
name -> Text,
}
}
table! {
posts {
id -> Integer,
by_user -> Nullable<Text>,
}
}
allow_tables_to_appear_in_same_query!(users, posts);
posts::table.filter(
posts::by_user.eq_any(users::table.select(users::name).nullable())
).load(connection)?;