24 days of Rust - postgres

Important note: this article is outdated! Go to http://zsiciarz.github.io/24daysofrust/ for a recent version of all of 24 days of Rust articles. The blogpost here is kept as it is for historical reasons.

Yes, I'm biased. PostgreSQL is my favorite SQL database and the upcoming 9.4 release makes me even more excited. There is already a pure Rust driver for PostgreSQL - the postgres crate which will be the subject of today's article.

Connecting

In this and the following examples we will assume a Postgres user rust with password rust and an existing database named... well, rust .

extern crate postgres ; use postgres :: { Connection , SslMode }; fn main () { let dsn = "postgresql://rust:rust@localhost/rust" ; let conn = match Connection :: connect ( dsn , & SslMode :: None ) { Ok ( conn ) => conn , Err ( e ) => { println ! ( "Connection error: {}" , e ); return ; } }; }

The Connection type has a few methods related to making queries; perhaps the simplest one is execute() which immediately executes the query and returns the number of modified rows (or an error). This method can be used for example for insert/update queries but also DDL as shown below.

conn . execute ( "create table if not exists blog ( id serial primary key, title varchar(255), body text)" , & []). ok (). expect ( "Table creation failed" );

The second argument looks slightly awkward, but that's the way of telling execute() that the query takes no parameters. Later on we will see a few examples that use query parameters.

Prepared queries and statements

Let's add a few rows to our table. We will use the prepare() method.

let stmt = match conn . prepare ( "insert into blog (title, body) values ($1, $2)" ) { Ok ( stmt ) => stmt , Err ( e ) => { println ! ( "Preparing query failed: {}" , e ); return ; } }; for i in range ( 1 , 5 u ) { let title = format ! ( "Blogpost number {}" , i ); let text = format ! ( "Content of the blogpost #{}" , i ); stmt . execute ( & [ & title , & text ]). ok (). expect ( "Inserting blogposts failed" ); }

The query was prepared only once and what we got (after some crude error handling) is a Statement value. We can use its execute() method to actually run the query with the supplied parameters (note the borrowing).

To read the data from the database we will use the same prepare() method in conjunction with the query() method of a Statement . There's a significant difference between execute() and query() : the former returns just the number of affected rows, while the latter returns a collection of Row values.

let stmt = match conn . prepare ( "select id, title, body from blog where id < $1" ) { Ok ( stmt ) => stmt , Err ( e ) => { println ! ( "Preparing query failed: {}" , e ); return ; } }; let max_id : i32 = 3 ; let mut rows = stmt . query ( & [ & max_id ]). ok (). expect ( "Selecting blogposts failed" ); for row in rows { let id : i32 = row . get ( "id" ); let title : String = row . get ( "title" ); println ! ( "ID={}, title={}" , id , title ); }

Keep in mind that the get() method will panic if it encounters incompatible types, for example if we changed String to i32 above. There's also a safer get_opt() method returning a Result instead of panicking.

Advanced PostgreSQL types

All this is a bit boring so far. One of the reasons developers love PostgreSQL is its selection of interesting data types. Let's see how to use them in Rust (hint: it's kinda cool). We'll start from writing a generic helper function to read a single value from the first column of the first row.

use postgres :: { Connection , Error , FromSql , SslMode }; use postgres :: Result as PgResult ; fn get_single_value < T > ( conn : & Connection , query : & str ) -> PgResult < T > where T : FromSql { println ! ( "Executing query: {}" , query ); let stmt = try ! ( conn . prepare ( query )); let mut rows = try ! ( stmt . query ( & [])); let row = try ! ( rows . next (). ok_or ( Error :: BadData )); row . get_opt ( 0 ) }

We use the try! macro to minimize the noise from error handling. Now let's see it in action.

println ! ( "{}" , get_single_value ::< bool > ( & conn , "select 1=1" )); println ! ( "{}" , get_single_value ::< i32 > ( & conn , "select 1=1" )); type IntArray = ArrayBase < Option < i32 >> ; let arr = get_single_value ::< IntArray > ( & conn , "select '{4, 5, 6}'::int[]" ); println ! ( "{}" , arr . map ( | arr | arr . values () . filter_map ( | x | * x ) // unwraps Some values and skips None . collect ::< Vec < _ >> ())); let json = get_single_value ::< Json > ( & conn , "select '{ \" foo \" : \" bar \" , \" answer \" : 42}'::json" ); println ! ( "{}" , json );

$ cargo run Executing query: select 1 = 1 Ok ( true ) Executing query: select 1 = 1 Err ( Unexpected type Bool ) Executing query: select '{4, 5, 6}' ::int [] Ok ([ 4, 5, 6 ]) Executing query: select '{"foo": "bar", "answer": 42}' ::json Ok ({ "answer" :42, "foo" : "bar" })

Fantastic! The error handling is still there when we need it and we get values of reasonable Rust types.

Compile-time SQL checking

There is another crate worth mentioning here - postgres_macros. It provides the sql! macro that validates correctness of the SQL query given as argument. See the readme for an example. Unfortunately I couldn't get it to install, but it looks like an interesting experiment.

See also

Code examples in this article were built with rustc 0.13.0-nightly and postgres 0.1.3.

Photo by Justin Ennis and shared under the Creative Commons Attribution 2.0 Generic License. See https://www.flickr.com/photos/averain/3101178552

Napisane 11 grudnia 2014