diesel-derive-enum

Use Rust enums directly with diesel ORM.

Diesel is great, but wouldn't it be nice if this would work?

use crate::schema::my_table;

pub enum MyEnum {
    Foo,
    Bar,
    BazQuxx,
}

fn do_some_work(data: MyEnum, connection: &mut Connection) {
    insert_into(my_table)
        .values(&data)
        .execute(connection)
        .unwrap();
}

Unfortunately, it won't work out of the box, because any type which we wish to use with Diesel must implement various traits. Tedious to do by hand, easy to do with a derive macro - enter diesel-derive-enum.

The latest release, 2.1.0, is tested against diesel 2.1.0 and rustc 1.65 (we try to keep in lock-step with diesel). For earlier versions of diesel, check out the 2.0.1 and 1.* releases of this crate.

Upgrading from 2.0.x -> 2.1.0

Using diesel-cli? Due to an upstream change, you may need to modify your existing diesel.toml file. As of version 2.1.0, it must contain the following lines:

[print-schema]
# ... other config ...
custom_type_derives = ["diesel::query_builder::QueryId"]

So if it doesn't - add it!

Setup with Diesel CLI

This crate integrates nicely with diesel-cli - this is the recommended workflow. Note that for now, this only works with Postgres - for other databases, or if not using Diesel CLI, see the next section.

Cargo.toml:

[dependencies]
diesel-derive-enum = { version = "2.1.0", features = ["postgres"] }

Suppose our project has the following diesel.toml (as generated by diesel setup):

[print_schema]
file = "src/schema.rs"
custom_type_derives = ["diesel::query_builder::QueryId"]

And the following SQL:

CREATE TYPE my_enum AS ENUM ('foo', 'bar', 'baz_quxx');

CREATE TABLE my_table (
  id SERIAL PRIMARY KEY,
  some_enum my_enum NOT NULL
);

Then running $ diesel migration run will generate code something like:

// src/schema.rs -- autogenerated

pub mod sql_types {
    #[derive(diesel::sql_types::SqlType, diesel::query_builder::QueryId)]
    #[diesel(postgres_type(name = "my_enum"))]
    pub struct MyEnum;
}

table! {
    use diesel::types::Integer;
    use super::sql_types::MyEnum;

    my_table {
        id -> Integer,
        some_enum -> MyEnum
    }
}

Now we can use diesel-derive-enum to hook in our own enum:

// src/my_code.rs

#[derive(diesel_derive_enum::DbEnum)]
#[ExistingTypePath = "crate::schema::sql_types::MyEnum"]
pub enum MyEnum {
    Foo,
    Bar,
    BazQuxx,
}

Note the ExistingTypePath attribute. This instructs this crate to import the (remote, autogenerated) type and implement various traits upon it. That's it! Now we can use MyEnum with diesel (see 'Usage' below).

Setup without Diesel CLI

If you are using mysql or sqlite, or you aren't using diesel-cli to generate your schema, the setup is a little different.

Postgres

Cargo.toml:

[dependencies]
diesel-derive-enum = { version = "2.1.0", features = ["postgres"] }

SQL:

CREATE TYPE my_enum AS ENUM ('foo', 'bar', 'baz_quxx');

CREATE TABLE my_table (
  id SERIAL PRIMARY KEY,
  some_enum my_enum NOT NULL
);

Rust:

#[derive(diesel_derive_enum::DbEnum)]
pub enum MyEnum {
    Foo,
    Bar,
    BazQuxx,
}

// define your table
table! {
    use diesel::types::Integer;
    use super::MyEnumMapping;
    my_table {
        id -> Integer,
        some_enum -> MyEnumMapping, // Generated Diesel type - see below for explanation
    }
}

MySQL

Cargo.toml:

[dependencies]
diesel-derive-enum = { version = "2.1.0", features = ["mysql"] }

SQL:

CREATE TABLE my_table (
    id SERIAL PRIMARY KEY,
    my_enum enum('foo', 'bar', 'baz_quxx') NOT NULL  -- note: snake_case
);

Rust:

#[derive(diesel_derive_enum::DbEnum)]
pub enum MyEnum {
    Foo,
    Bar,
    BazQuxx,
}

// define your table
table! {
    use diesel::types::Integer;
    use super::MyEnumMapping;
    my_table {
        id -> Integer,
        some_enum -> MyEnumMapping, // Generated Diesel type - see below for explanation
    }
}

sqlite

Cargo.toml:

[dependencies]
diesel-derive-enum = { version = "2.1.0", features = ["sqlite"] }

SQL:

CREATE TABLE my_table (
    id SERIAL PRIMARY KEY,
    my_enum TEXT CHECK(my_enum IN ('foo', 'bar', 'baz_quxx')) NOT NULL   -- note: snake_case
);

Rust:

#[derive(diesel_derive_enum::DbEnum)]
pub enum MyEnum {
    Foo,
    Bar,
    BazQuxx,
}

// define your table
table! {
    use diesel::types::Integer;
    use super::MyEnumMapping;
    my_table {
        id -> Integer,
        some_enum -> MyEnumMapping, // Generated Diesel type - see below for explanation
    }
}

Usage

Once set up, usage is similar regardless of your chosen database. We can define a struct with which to populate/query the table:

#[derive(Insertable, Queryable, Identifiable, Debug, PartialEq)]
#[diesel(table_name = my_table)]
struct  MyRow {
    id: i32,
    some_enum: MyEnum,
}

And use it in the natural way:

let data = vec![
    MyRow {
        id: 1,
        some_enum: MyEnum::Foo,
    },
    MyRow {
        id: 2,
        some_enum: MyEnum::BazQuxx,
    },
];
let connection = PgConnection::establish(/*...*/).unwrap();
let inserted = insert_into(my_table::table)
    .values(&data)
    .get_results(&connection)
    .unwrap();
assert_eq!(data, inserted);

Postgres arrays work too! See this example.

Enums Representations

Enums are not part of the SQL standard and have database-specific implementations.

• In Postgres, we declare an enum as a separate type within a schema (CREATE TYPE ...), which may then be used in multiple tables. Internally, an enum value is encoded as an int (four bytes) and stored inline within a row (a much more efficient representation than a string).

• MySQL is similar except the enum is not declared as a separate type and is 'local' to it's parent table. It is encoded as either one or two bytes.

• sqlite does not have enums - in fact, it does not really have types; you can store any kind of data in any column. Instead we emulate static checking by adding the CHECK command, as per above. This does not give a more compact encoding but does ensure better data integrity. Note that if you somehow retreive some other invalid text as an enum, diesel will error at the point of deserialization.

How It Works

Diesel maintains a set of internal types which correspond one-to-one to the types available in various relational databases. Each internal type in turn maps to some kind of Rust native type. e.g. Postgres INTEGER maps to diesel::types::Integer maps to i32.

For postgres only, as of diesel-2.0.0, diesel-cli will create the 'dummy' internal enum mapping type as part of the schema generation process. We then specify the location of this type with the ExistingTypePath attribute.

In the case where ExistingTypePath is not specified, we assume the internal type has not already been generated, so this macro will instead create it with the default name {enum_name}Mapping. This name can be overridden with the DieselType attribute.

In either case, this macro will then implement various traits on the internal type. This macro will also implement various traits on the user-defined enum type. The net result of this is that the user-defined enum can be directly inserted into (and retrieved from) the diesel database.

Note that by default we assume that the possible SQL ENUM variants are simply the Rust enum variants translated to snake_case. These can be renamed with the inline annotation #[db_rename = "..."].

See this test for an example of renaming.

You can override the snake_case assumption for the entire enum using the #[DbValueStyle = "..."] attribute. Individual variants can still be renamed using #[db_rename = "..."].

See this test for an example of changing the output style.

License

Licensed under either of these:

• Apache License, Version 2.0, (LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or https://opensource.org/licenses/MIT)