Function diesel::dsl::copy_to

source ·
pub fn copy_to<T>(target: T) -> CopyToQuery<T, NotSet>
where T: CopyTarget,
Available on crate feature postgres_backend only.
Expand description

Creates a COPY TO statement

This function constructs a COPY TO statement which copies data from the database to a client side target. It’s designed to move larger amounts of data out of the database.

This function accepts a target selection (table name or list of columns) as argument.

There are two ways to use a COPY TO statement with diesel:

  • By using CopyToQuery::load directly to load the deserialized result directly into a specified type
  • By using the with_* methods to configure the format sent by the database and then by calling CopyToQuery::load_raw to receive the raw data sent by the database.

The first variant uses the BINARY format internally to receive the selected data efficiently. It automatically sets the right options and does not allow to change them via with_* methods.

The second variant allows you to control the behaviour of the generated COPY TO statement in detail. You can use the various with_* methods for that before issuing the statement via CopyToQuery::load_raw. That method will return an type that implements std::io::BufRead, which allows you to directly read the response from the database in the configured format. See the postgresql documentation for more details about the supported formats.

If you don’t have any specific needs you should prefer using the more convenient first variant.

This functionality is postgresql specific.


§Via CopyToQuery::load()

#[derive(Queryable, Selectable, PartialEq, Debug)]
#[diesel(table_name = users)]
struct User {
    name: String,

let out = diesel::copy_to(users::table)
    .load::<User, _>(connection)?
    .collect::<Result<Vec<_>, _>>()?;

assert_eq!(out, vec![User{ name: "Sean".into() }, User{ name: "Tess".into() }]);

§Via CopyToQuery::load_raw()

use diesel::pg::CopyFormat;
use std::io::Read;

let mut copy = diesel::copy_to(users::table)

let mut out = String::new();
copy.read_to_string(&mut out).unwrap();
assert_eq!(out, "1,Sean\n2,Tess\n");