[Rust Diesel] Getting Started - Diesel CRUD in PostgreSQL

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Diesel

Diesel is a Safe, Extensible ORM and Query Builder for Rust.

Diesel is the most productive way to interact with databases in Rust because of its safe and composable abstractions over queries.

For this guide, we’re going to walk through some simple examples for each of the pieces of CRUD, which stands for “Create Read Update Delete”. Each step in this guide will build on the previous, and is meant to be followed along.

Prerequisites

  • PostgreSQL

    Make sure you have PostgreSQL installed and running.

    PostgreSQL: Downloads - https://www.postgresql.org/download/

  • Rust versions

    Diesel requires Rust 1.31 or later. If you’re following along with this guide, make sure you’re using at least
    that version of Rust by running rustup update stable.

Usages

Initializing a new project

The first thing we need to do is generate our project.

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# Generate a new project
cargo new --lib diesel_demo
cd diesel_demo
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# Cargo.toml

[dependencies]
diesel = { version = "1.4.4", features = ["postgres"] }
dotenv = "0.15.0"

Installing Diesel CLI

Diesel provides a separate CLI tool to help manage your project. Since it’s a standalone binary, and doesn’t affect your project’s code directly, we don’t add it to Cargo.toml. Instead, we just install it on our system.

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# Install the CLI tool.
$ cargo install diesel_cli

# Install diesel_cli only PostgreSQL.
# $ cargo install diesel_cli --no-default-features --features postgres

Create Database

We need to create a database with username and password.

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-- psql

CREATE USER diesel_demo WITH PASSWORD 'diesel_demo';

CREATE DATABASE diesel_demo OWNER diesel_demo;

GRANT ALL PRIVILEGES ON DATABASE diesel_demo TO diesel_demo;

Setup Diesel for your project

We need to tell Diesel where to find our database. We do this by setting the DATABASE_URL environment variable. On our development machines, we’ll likely have multiple projects going, and we don’t want to pollute our environment. We can put the url in a .env file instead.

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$ echo DATABASE_URL=postgres://diesel_demo:diesel_demo@localhost/diesel_demo > .env

Now Diesel CLI can set everything up for us.

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$ diesel setup

This will create our database (if it didn’t already exist), and create an empty migrations directory that we can use to manage our schema (more on that later).

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$ tree migrations 
migrations
└── 00000000000000_diesel_initial_setup
    ├── down.sql
    └── up.sql

The first thing we’re going to need is a table to store our posts. Let’s create a migration.

Diesel CLI will create two empty files for us in the required structure. You’ll see output that looks something like this:

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$ diesel migration generate create_posts
Creating migrations/20210626133237_create_posts/up.sql
Creating migrations/20210626133237_create_posts/down.sql

Migrations allow us to evolve the database schema over time. Each migration can be applied (up.sql) or reverted (down.sql). Applying and immediately reverting a migration should leave your database schema unchanged.

Next, we’ll write the SQL for migrations:

up.sql

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CREATE TABLE posts (
  id SERIAL PRIMARY KEY,
  title VARCHAR NOT NULL,
  body TEXT NOT NULL,
  published BOOLEAN NOT NULL DEFAULT 'f'
);

down.sql

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DROP TABLE posts;

We can apply our new migration:

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$ diesel migration run
Running migration 2021-06-26-133237_create_posts

It’s a good idea to make sure that down.sql is correct. You can quickly confirm that your down.sql rolls back your migration correctly by redoing the migration:

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$ diesel migration redo
Rolling back migration 2021-06-26-133237_create_posts
Running migration 2021-06-26-133237_create_posts

A Note on Raw SQL in Migrations:

Since migrations are written in raw SQL, they can contain specific features of the database system you use. For example, the CREATE TABLE statement above uses PostgreSQL’s SERIAL type. If you want to use SQLite instead, you need to use INTEGER instead.

A Note on Using Migrations on fly:

The diesel_migrations diesel_migrations 1.4.0 - Docs.rs - https://docs.rs/crate/diesel_migrations/1.4.0 crate provides the embed_migrations! macro, allowing you to embed migration scripts in the final binary. Once your code uses it, you can simply include embedded_migrations::run(&db_conn) at the start of your main function to run migrations every time the application starts.

Write Rust

We’ll start by writing some code to show the last five published posts. The first thing we need to do is establish a database connection.

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// src/lib.rs

pub mod schema;
pub mod models;

#[macro_use]
extern crate diesel;
extern crate dotenv;

use diesel::prelude::*;
use diesel::pg::PgConnection;
use dotenv::dotenv;
use std::env;

pub fn establish_connection() -> PgConnection {
    dotenv().ok();

    let database_url = env::var("DATABASE_URL")
        .expect("DATABASE_URL must be set");
    PgConnection::establish(&database_url)
        .expect(&format!("Error connecting to {}", database_url))
}

Next we need to create the two modules that we just declared.

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// src/models.rs

#[derive(Queryable)]
pub struct Post {
    pub id: i32,
    pub title: String,
    pub body: String,
    pub published: bool,
}

#[derive(Queryable)] will generate all of the code needed to load a Post struct from a SQL query.

Typically the schema module isn’t created by hand, it gets generated by Diesel. When we ran diesel setup, a file called diesel.toml was created which tells Diesel to maintain a file at src/schema.rs for us. The file should look like this:

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// src/schema.rs

table! {
    posts (id) {
        id -> Integer,
        title -> Text,
        body -> Text,
        published -> Bool,
    }
}

The exact output might vary slightly depending on your database, but it should be equivalent.

The table! macro creates a bunch of code based on the database schema to represent all of the tables and columns. We’ll see how exactly to use that in the next example.

Any time we run or revert a migration, this file will get automatically updated.

A Note on Field Order

Using #[derive(Queryable)] assumes that the order of fields on the Post struct matches the columns in the posts table, so make sure to define them in the order seen in the schema.rs file.

Show Posts

Let’s write the code to actually show us our posts.

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// src/bin/show_posts.rs

extern crate diesel_demo;
extern crate diesel;

use self::diesel_demo::*;
use self::models::*;
use self::diesel::prelude::*;

fn main() {
    use diesel_demo::schema::posts::dsl::*;

    let connection = establish_connection();
    let results = posts.filter(published.eq(true))
        .limit(5)
        .load::<Post>(&connection)
        .expect("Error loading posts");

    println!("Displaying {} posts", results.len());
    for post in results {
        println!("{}", post.title);
        println!("----------\n");
        println!("{}", post.body);
    }
}

The use diesel_demo::schema::posts::dsl::* line imports a bunch of aliases so that we can say posts instead of posts::table, and published instead of posts::published. It’s useful when we’re only dealing with a single table, but that’s not always what we want.

We can run our script with cargo run --bin show_posts. Unfortunately, the results won’t be terribly interesting, as we don’t actually have any posts in the database. Still, we’ve written a decent amount of code, so let’s commit.

The full code for the demo at this point can be found here diesel/examples/postgres/getting_started_step_1 at v1.4.4 · diesel-rs/diesel - https://github.com/diesel-rs/diesel/tree/v1.4.4/examples/postgres/getting_started_step_1/.

Create Post

Next, let’s write some code to create a new post.We’ll want a struct to use for inserting a new record.

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// src/models.rs

use super::schema::posts;

#[derive(Insertable)]
#[table_name="posts"]
pub struct NewPost<'a> {
    pub title: &'a str,
    pub body: &'a str,
}

Now let’s add a function to save a new post.

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// src/lib.rs

use self::models::{Post, NewPost};

pub fn create_post<'a>(conn: &PgConnection, title: &'a str, body: &'a str) -> Post {
    use schema::posts;

    let new_post = NewPost {
        title: title,
        body: body,
    };

    diesel::insert_into(posts::table)
        .values(&new_post)
        .get_result(conn)
        .expect("Error saving new post")
}

When we call .get_result on an insert or update statement, it automatically adds RETURNING * to the end of the query, and lets us load it into any struct that implements Queryable for the right types. Neat!

Diesel can insert more than one record in a single query. Just pass a Vec or slice to insert, and then call get_results instead of get_result. If you don’t actually want to do anything with the row that was just inserted, call .execute instead. The compiler won’t complain at you, that way. :)

Now that we’ve got everything set up, we can create a little script to write a new post.

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// src/bin/write_post.rs

extern crate diesel_demo;
extern crate diesel;

use self::diesel_demo::*;
use std::io::{stdin, Read};

fn main() {
    let connection = establish_connection();

    println!("What would you like your title to be?");
    let mut title = String::new();
    stdin().read_line(&mut title).unwrap();
    let title = &title[..(title.len() - 1)]; // Drop the newline character
    println!("\nOk! Let's write {} (Press {} when finished)\n", title, EOF);
    let mut body = String::new();
    stdin().read_to_string(&mut body).unwrap();

    let post = create_post(&connection, title, &body);
    println!("\nSaved draft {} with id {}", title, post.id);
}

#[cfg(not(windows))]
const EOF: &'static str = "CTRL+D";

#[cfg(windows)]
const EOF: &'static str = "CTRL+Z";

We can run our new script with cargo run --bin write_post. Go ahead and write a blog post. Get creative! Here was mine:

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$ cargo run --bin write_post
   Compiling diesel_demo v0.1.0 (file:///Users/cloudolife/diesel_demo)
     Running `target/debug/write_post`

What would you like your title to be?
Diesel demo

Ok! Let's write Diesel demo (Press CTRL+D when finished)

You know, a CLI application probably isn't the best interface for a blog demo.
But really I just wanted a semi-simple example, where I could focus on Diesel.
I didn't want to get bogged down in some web framework here.
Plus I don't really like the Rust web frameworks out there. We might make a
new one, soon.

Saved draft Diesel demo with id 1

Unfortunately, running show_posts still won’t display our new post, because we saved it as a draft. If we look back to the code in show_posts, we added .filter(published.eq(true)), and we had published default to false in our migration. We need to publish it! But in order to do that, we’ll need to look at how to update an existing record. First, let’s commit. The code for this demo at this point can be found here.

Publish post

Now that we’ve got create and read out of the way, update is actually relatively simple. Let’s jump right into the script:

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// src/bin/publish_post.rs

extern crate diesel_demo;
extern crate diesel;

use self::diesel::prelude::*;
use self::diesel_demo::*;
use self::models::Post;
use std::env::args;

fn main() {
    use diesel_demo::schema::posts::dsl::{posts, published};

    let id = args().nth(1).expect("publish_post requires a post id")
        .parse::<i32>().expect("Invalid ID");
    let connection = establish_connection();

    let post = diesel::update(posts.find(id))
        .set(published.eq(true))
        .get_result::<Post>(&connection)
        .expect(&format!("Unable to find post {}", id));
    println!("Published post {}", post.title);
}

That’s it! Let’s try it out with cargo run --bin publish_post 1.

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$ cargo run --bin publish_post 1
 Compiling diesel_demo v0.1.0 (file:///Users/cloudolife/diesel_demo)
   Running `target/debug/publish_post 1`
Published post Diesel demo
And now, finally, we can see our post with cargo run --bin show_posts.

     Running `target/debug/show_posts`
Displaying 1 posts
Diesel demo
----------

You know, a CLI application probably isn't the best interface for a blog demo.
But really I just wanted a semi-simple example, where I could focus on Diesel.
I didn't want to get bogged down in some web framework here.
Plus I don't really like the Rust web frameworks out there. We might make a
new one, soon.

We’ve still only covered three of the four letters of CRUD though. Let’s show how to delete things. Sometimes we write something we really hate, and we don’t have time to look up the ID. So let’s delete based on the title, or even just some words in the title.

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// src/bin/delete_post.rs

extern crate diesel_demo;
extern crate diesel;

use self::diesel::prelude::*;
use self::diesel_demo::*;
use std::env::args;

fn main() {
    use diesel_demo::schema::posts::dsl::*;

    let target = args().nth(1).expect("Expected a target to match against");
    let pattern = format!("%{}%", target);

    let connection = establish_connection();
    let num_deleted = diesel::delete(posts.filter(title.like(pattern)))
        .execute(&connection)
        .expect("Error deleting posts");

    println!("Deleted {} posts", num_deleted);
}

We can run the script with cargo run --bin delete_post demo (at least with the title I chose). Your output should look something like:

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$ cargo run --bin delete_post
   Compiling diesel_demo v0.1.0 (file:///Users/cloudolife/diesel_demo)
     Running `target/debug/delete_post demo`
Deleted 1 posts

When we try to run cargo run --bin show_posts again, we can see that the post was in fact deleted. This barely scratches the surface of what you can do with Diesel, but hopefully this tutorial has given you a good foundation to build off of. We recommend exploring the API docs to see more. The final code for this tutorial can be found here diesel/examples/postgres/getting_started_step_3 at v1.4.4 · diesel-rs/diesel - https://github.com/diesel-rs/diesel/tree/v1.4.4/examples/postgres/getting_started_step_3/.

FAQs

cannot find macro table in this scope, cannot find derive macro Queryable in this scope

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$ cargo build
...
error: cannot find macro `table` in this scope
 --> src/schema.rs:1:1
  |
1 | table! {
  | ^^^^^
  |
  = note: consider importing this macro:
          diesel::table

error: cannot find derive macro `Queryable` in this scope
 --> src/models.rs:2:10
  |
2 | #[derive(Queryable)]
  |          ^^^^^^^^^

error: aborting due to 2 previous errors

error: could not compile `diesel_demo`
...
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// src/lib.rs

// Enable use diesel macro
#[macro_use]
extern crate diesel;

References

[1] Getting Started - https://diesel.rs/guides/getting-started.html

[2] Diesel is a Safe, Extensible ORM and Query Builder for Rust - https://diesel.rs/

[3] diesel/examples/postgres/getting_started_step_3 at v1.4.4 · diesel-rs/diesel - https://github.com/diesel-rs/diesel/tree/v1.4.4/examples/postgres/getting_started_step_3/

[4] diesel - Rust - https://docs.diesel.rs/master/diesel/index.html

[5] rustup.rs - The Rust toolchain installer - https://rustup.rs/

[6] PostgreSQL: Downloads - https://www.postgresql.org/download/