A classy high-level Clojure library for defining application models and retrieving them from a DB
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camsaul Merge pull request #39 from metabase/update-dep
Update deps & bump version -> 1.1.9
Latest commit addfcef Jul 13, 2018


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There are no SQL/Relational DB ORMs for Clojure for obvious reasons. -- Andrew Brehaut

Toucan provides the better parts of an ORM for Clojure, like simple DB queries, flexible custom behavior when inserting or retrieving objects, and easy hydration of related objects, all in a powerful and classy way.

Toucan builds on top of clojure.java.jdbc and the excellent HoneySQL. The code that inspired this library was originally written to bring some of the sorely missed conveniences of Korma to HoneySQL when we transitioned from the former to the latter at Metabase. Over the last few years, I've continued to build upon and refine the interface of Toucan, making it simpler, more powerful, and more flexible, all while maintaining the function-based approach of HoneySQL.

View the complete documentation here, or continue below for a brief tour.

Simple Queries:

Toucan greatly simplifies the most common queries without limiting your ability to express more complicated ones. Even something relatively simple can take quite a lot of code to accomplish in HoneySQL:

clojure.java.jdbc + HoneySQL

;; select the :name of the User with ID 100
(-> (jdbc/query my-db-details
         {:select [:name]
          :from   [:user]
          :where  [:= :id 100]
          :limit  1}
         :quoting :ansi))


;; select the :name of the User with ID 100
(db/select-one-field :name User :id 100)

Toucan keeps the simple things simple. Read more about Toucan's database functions here.

Flexible custom behavior when inserting and retrieving objects:

Toucan makes it easy to define custom behavior for inserting, retrieving, updating, and deleting objects on a model-by-model basis. For example, suppose you want to convert the :status of a User to a keyword whenever it comes out of the database, and back into a string when it goes back in:

clojure.java.jdbc + HoneySQL

;; insert new-user into the DB, converting the value of :status to a String first
(let [new-user {:name "Cam", :status :new}]
  (jdbc/insert! my-db-details :user (update new-user :status name)))

;; fetch a user, converting :status to a Keyword
(-> (jdbc/query my-db-details
        {:select [:*]
         :from   [:user]
         :where  [:= :id 200]}))
    (update :status keyword))


With Toucan, you just need to define the model, and tell that you want :status automatically converted:

;; define the User model
(defmodel User :user
  (types [this] ;; tell Toucan to automatically do Keyword <-> String conversion for :status
    {:status :keyword}))

After that, whenever you fetch, insert, or update a User, :status will automatically be converted appropriately:

;; Insert a new User
(db/insert! User :name "Cam", :status :new) ; :status gets stored in the DB as "new"

;; Fetch User 200
(User 200) ; :status is converted to a keyword when User is fetched

Read more about defining and customizing the behavior of models here.

Easy hydration of related objects

If you're developing something like a REST API, there's a good chance at some point you'll want to hydrate some related objects and return them in your response. For example, suppose we wanted to return a Venue with its Category hydrated:

;; No hydration
{:name        "The Tempest"
 :category_id 120

;; w/ hydrated :category
{:name        "The Tempest"
 :category_id 120
 :category    {:name "Dive Bar"

The code to do something like this is enormously hairy in vanilla JDBC + HoneySQL. Luckily, Toucan makes this kind of thing a piece of cake:

(hydrate (Venue 120) :category)

Toucan is clever enough to automatically figure out that Venue's :category_id corresponds to the :id of a Category, and constructs efficient queries to fetch it. Toucan can hydrate single objects, sequences of objects, and even objects inside hydrated objects, all in an efficient way that minimizes the number of DB calls. You can also define custom functions to hydrate different keys, and add additional mappings for automatic hydration. Read more about hydration here.

Getting Started

To get started with Toucan, all you need to do is tell it how to connect to your DB by calling set-default-db-connection!:

(require '[toucan.db :as db])

  {:classname   "org.postgresql.Driver"
   :subprotocol "postgresql"
   :subname     "//localhost:5432/my_db"
   :user        "cam"})

Pass it the same connection details that you'd pass to any clojure.java.jdbc function; it can be a simple connection details map like the example above or some sort of connection pool. This function only needs to be called once, and can done in your app's normal entry point (such as -main) or just as a top-level function call outside any other function in a namespace that will get loaded at launch.

Read more about setting up the DB and configuring options such as identifier quoting style here.

Test Utilities

Toucan provides several utility macros that making writing tests easy. For example, you can easily create temporary objects so your tests don't affect the state of your test DB:

(require '[toucan.util.test :as tt])

;; create a temporary Venue with the supplied values for use in a test.
;; the object will be removed from the database afterwards (even if the macro body throws an Exception)
;; which makes it easy to write tests that don't change the state of the DB
  (tt/with-temp Venue [venue {:name "Ho's Bootleg Tavern"}]
    (venue-slug venue))

Read more about Toucan test utilities here.

Annotated Source Code

View the annotated source code here, generated with Marginalia.


Pull requests for bugfixes, improvements, more documentaton, and other enhancements are always welcome. Toucan code is written to the strict standards of the Metabase Clojure Style Guide, so take a moment to familiarize yourself with the style guidelines before submitting a PR.

If you're interested in contributing, be sure to check out issues tagged "help wanted". There's lots of ways Toucan can be improved and we need people like you to make it even better.

Tests & Linting

Before submitting a PR, you should also make sure tests and the linters pass. You can run tests and linters as follows:

lein test && lein lint

Tests assume you have Postgres running locally and have a test DB set up with read/write permissions. Toucan will populate this database with appropriate test data.

If you don't have Postgres running locally, you can instead the provided shell script to run Postgres via docker. Use lein start-db to start the database and lein stop-db to stop it. Note the script is a Bash script and won't work on Windows unless you're using the Windows Subsystem for Linux.

To configure access to this database, set the following env vars as needed:

Env Var Default Value Notes
TOUCAN_TEST_DB_USER Optional when running Postgres locally
TOUCAN_TEST_DB_PASS Optional when running Postgres locally

These tests and linters also run on CircleCI for all commits and PRs.

We try to keep Toucan well-tested, so new features should include new tests for them; bugfixes should include failing tests. Make sure to properly document new features as well! 😋

A few more things: please carefully review your own changes and revert any superfluous ones. (A good example would be moving words in the Markdown documentation to different lines in a way that wouldn't change how the rendered page itself would appear. These sorts of changes make a PR bigger than it needs to be, and, thus, harder to review.) And please include a detailed explanation of what changes you're making and why you've made them. This will help us understand what's going on while we review it. Thanks! 😻


Code and documentation copyright © 2018 Metabase, Inc. Artwork copyright © 2018 Cam Saul.

Distributed under the Eclipse Public License, same as Clojure.