Diesel 2.0.0

Diesel 2.0.0 contains the contributions of more than 130 people. More than 1700 commits were submitted over a span of 3 years.

As part of this release we introduced numerous new features and rewrote large parts of the internal structure. Check out our changelog for a complete list of changes. As this is a new major Diesel release it contains a number of breaking changes. Checkout our migration guide for details about how to handle those breaking changes.

This release contains the following parts:

diesel 2.0.0
diesel_derives 2.0.0
diesel_migrations 2.0.0
diesel_cli 2.0.0
diesel_dynamic_schema 0.2.0

Support the development of Diesel by sponsoring our work on GitHub

Features

As a highlight Diesel 2.0.0 adds support for the following features:

Fully type checked GROUP BY support
Support for table aliasing
Support for defining select clauses via a corresponding type
Support for UNION/INTERSECT queries

In addition to the highlighted features Diesel 2.0.0 fixes several issues in our type level SQL representation such that it now correctly handles the following cases:

Mixed nested LEFT JOINS and INNER JOINS
Chaining mixed nullable expressions via AND, OR and similar operators

Support for `GROUP BY` clauses

Diesel 2.0 adds support for GROUP BY clauses for select queries.

This means queries like the following one will just work.

Example

 users::table.inner_join(posts::table)
    .group_by(users::id)
    .select((users::name, count(posts::id)))

As this is the case for all other Diesel built-in query dsl, this construct is fully checked at compile time. This means Diesel will ensure that the GROUP BY clause is valid for the current query and it will also ensure that expressions appearing inside of your SELECT clause will match the aggregation rules provided by the current GROUP BY clause. Checkout the documentation of QueryDsl::group_by for examples.

Support for table aliasing

Diesel 2.0 adds support for table aliasing. This enables users to write queries, where a table appears more than once in the corresponding FROM clause. For this Diesel provides a diesel::alias! macro that allows to define new alias for existing tables.

The following query demonstrates the support for this feature:

Example

// Define new table alias for the existing `users` table
let users1 = diesel::alias!(schema::users as user1);

// Use the corresponding alias inside any existing query
users::table
    .inner_join(users1.on(users::id).eq(users1.field(users::id))))
    .select((users::id, users::name, users1.field(users::name)))
    .order_by(users1.field(users::id))

Again all of this is checked at compile time. So similar to a normal table, columns from aliases are only allowed to appear if the corresponding query actually uses the alias.

Implied selection via the new `Selectable` trait

Diesel 2.0 features a new Selectable trait and derive that lets users declare that a type expects a certain kind of select clause. The major use case for this feature is to ensure that columns from a specific query are always requested in the right order for a corresponding type implementing Queryable. This also works for complex queries involving joins or other kinds of nesting.

Example

#[derive(Queryable, Selectable)]
struct User {
    id: i32,
    name: String,
}

let first_user = users.select(User::as_select()).first(connection)?;

Diesel enforces at type system level that once you provided such a select clause via User::as_select() you are only allowed to construct this type from the returned result of the corresponding query. This means there is no need to specify the User type twice in the query above.

Support for `UNION`/`INTERSECT`/`EXCEPT` queries

Diesel 2.0 extents the query builder to support query combinations via UNION/INTERSECT/EXCEPT. This allows you to easily chain multiple queries together as long as they return fields of the same type. Queries like the following one are now supported:

Example

 users.select(user_name.nullable())
    .union(animals.select(animal_name).filter(animal_name.is_not_null()))

As always this is checked at compile time to reject invalid queries, like for example that ones containing select clauses with different fields. Checkout the documentation of CombineDsl for details.

Call for Participation

The release of Diesel 2.0 does not only include the features listed above, but also marks the point where the following things can be provided by third party crates:

Custom QueryDsl extensions to support previously unsupported SQL features. Checkout diesel_full_text_search for an example
Alternative query dsl implementations reusing the existing Connection infrastructure
Custom Connection implementations for existing backends
Custom Connection and Backend implementations for previously unsupported backends. Checkout diesel-oci for an example.

We encourage our community to try out those features. Especially we would like to see experimentation around:

Previously unsupported database backends
Pure rust implementations of existing backend implementations. Checkout this and this discussion for starting points.
Alternative query dsl implementations. Checkout this discussion as starting point.

Please get in touch with us for pointers, help and details.

Input for Future Roadmap

With the release of Diesel 2.0 the planing for our next releases start. Hopefully they will not take as long as Diesel 2.0. We are looking for input on which features are wanted by our community. Please open a discussion thread with your idea in our discussion forum.

Weiznich will work on improving error messages for trait heavy crates based on a Rust Foundation Project Grant. This work will hopefully improve error messages for Diesel as well. If you are aware of bad error messages please submit a minimal example here.

Update considerations

Diesel 2.0 introduces substantial changes to Diesel’s inner workings. In some cases this impacts code written using Diesel 1.4.x. This document outlines notable changes and presents potential update strategies. We recommend to start the upgrade by removing the usage of all items that are marked as deprecated in Diesel 1.4.x.

Any code base migrating from Diesel 1.4.x to Diesel 2.0 is expected to be affected at least by the following changes:

Users of diesel_migration are additionally affected by the following change:

diesel_migration rewrite

Users of BoxableExpression might be affected by the following change:

Changed nullability of operators

Users of tables containing a column of the type Array<T> are affected by the following change:

Changed nullability of array elements

Users that implement support for their SQL types or type mappings are affected by the following changes:

no_arg_sql_function! macro is now pending deprecation. Users of the macro are advised to consider sql_function! macro.

Deprecated usage of no_arg_sql_function! macro

Users of eq_any on the PostgreSQL backend might hit type rejection error in rare cases.

Changed accepted argument to eq_any() for the PostgreSQL backend

Users that update generic Diesel code will also be affected by the following changes:

Additionally this release contains many changes for users that implemented a custom backend/connection. We do not provide explicit migration steps but we encourage users to reach out with questions pertaining to these changes.

Thanks

As part of this release we would like to welcome @Ten0 as part of the diesel core team.

Thank you to everyone who helped make this release happen through sponsoring, bug reports, and discussion on GitHub and Gitter. While we don’t have a way to collect stats on that form of contribution, it’s greatly appreciated.

In addition to the Diesel core team, 141 people contributed code to this release. A huge thank you to:

Alessandro Menezes
Alexander ‘z33ky’ Hirsch
Alexei Pastuchov
Alice Ryhl
Amila Welihinda
Andre Braga Reis
Andreas Runfalk
Andrew Safigan
Andrew Speed
Andy Russell
Artem Vorotnikov
Arve Seljebu
Billy Chan
Blas Rodriguez Irizar
Bryan Henry
Callym
Caroline Glassberg-Powell
Cassie Jones
Chenxi Yuan
Chris Eckhardt
Chris Hanks
Chris Maddox
Chris West (Faux)
Clouds Flowing
Corentin Henry
Daniel Buse
Danilo Bargen
David Teller
David Tulig
DebugSteven
Diggory Blake
Dmitriy Pleshevskiy
Dusty Mabe
DrVilepis
EclipsedSolari
Emile Fugulin
Emm
Emmanuel Surleau
Erlend Langseth
Felix Watts
Filip Gospodinov
Garrett Thornburg
Giorgio Gambino
Grégory Obanos
Hal Gentz
Han Xu
Heliozoa
Henk van der Laan
Henry Boisdequin
Hirokazu Hata
Iban Eguia (Razican)
Igor Raits
Ivan Tham
JR Heard
Jean SIMARD
Jeremy Stucki
Jiří Sejkora
jigaoqiang
Joel Parker Henderson
John Brandt
Jonas Platte
Jonas Schievink
Joshua Koudys
Juhasz Sandor
Justice4Joffrey
Katharina Fey
Kevin King
Kevin Kirchner
Khionu Sybiern
Kitsu
Koisell
Kononnable
Leonardo Yvens
Lukas Markeffsky
Maccesch
Marc-Stefan Cassola
Martell Malone
Martijn Groeneveldt
Martin Nordholts
Matthew Kuo
Matthieu Guillemot
Mcat12
Meven
Mike Cronce
Mr Ceperka
Nafiul Islam
Nathan Papapietro
Nicholas Yang
Oliver Cooper
Otto Castle
Pankaj Jangid
Paolo Barbolini
Paul Le Corre
Paul Martensen
Pavan Kumar Sunkara
Paweł Przeniczny
Philip Trauner
Raphael Arias
Roman
Ryan Leckey
Sarthak Singh
Scott Driggers
Sean Klein
Simon Ertl
Spencer Taylor
Steven Chu
Storm Timmermans
Sébastien Santoro
Takayuki Maeda
Thomas Constantine Moore
Thomas Eizinger
Thomas Etter
Tom MacWright
Tuetuopay
Urhengulas
Vanio Begic
WebeWizard
William Myers
Yin Jifeng
Yuki Okushi
Zane Duffield
blackghost1987
czotomo
dchenk
ejc Drobnič
gorbit99
hasezoey
hi-rustin
kevinpoitra
kpcyrd
matthew-dowdell
ode79
ropottnik
telios
theredfish
zoewithabang
Zhenhui Xie
Émile Fugulin
κeen
二手掉包工程师
棒棒彬_Binboy

Diesel 2.0.0

Features

Support for GROUP BY clauses

Support for table aliasing

Implied selection via the new Selectable trait

Support for UNION/INTERSECT/EXCEPT queries

Call for Participation

Input for Future Roadmap

Update considerations

Thanks

Support for `GROUP BY` clauses

Implied selection via the new `Selectable` trait

Support for `UNION`/`INTERSECT`/`EXCEPT` queries