A relational mapper in Clojure
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Relational Mapper

A relational mapper in Clojure. If you're using relational database in Clojure then this library is for you.

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Make calls like:

(find-all db-state :posts #{:authors :attachments} [:= post.id 1])

and get results like:

    {:title "Christmas"
     :body "Merry Christmas!"
     :id 1
     :authors_id 10
     :authors {:name "Rudolf" :id 10}
     :attachments [{:name "rudolf.png" :id 100 :posts_id 1}
                   {:name "santa.png" :id 101 :posts_id 1}]

to achieve that though you have to first tell 'relational-mapper' what's the structure of your data and how to connect to the database, so the full, working example would look like this:

(ns your-project
  (:require [relational-mapper :refer :all]
            [relational-mapper.data-model :as data-model]))

(def associations {:authors {:posts {:type :has-many}
                             :attachments {:type :has-many :through :posts}}
                   :posts {:authors {:type :belongs-to}
                           :attachments {:type :has-many}}
                   :attachments {:authors {:type :belongs-to :through :posts}
                                 :posts {:type :belongs-to}}})

(def db-config {:classname "org.postgresql.Driver"
                :subprotocol "postgresql"
                :subname (str "//" "localhost" ":" 5432 "/" "testdb")
                :user "postgres-user"
                :password "postgres-password"})

(def initial-db-state {:config db-config
                       :data-model {})

(def db-state (data-model/set-associations initial-db-state associations {})

(find-all db-state :posts #{:authors :attachments} [:= post.id 1])

How to define associations

relational-mapper uses the same relations naming as 'Ruby On Rails`' 'ActiveRecord', which means:

  • posts has-many attachments means that attachments has posts_id column that refers to id column of posts table and there might be more than one attachment for one user (hence in response of find-all function, attachments is an array of hashes)

  • attachments has-one file means that file has attachments_id column that refers to id column in attachments table. There can be only one file per attachment though (so find-all will return files as a hash and not an array)

  • attachments belongs-to posts means that attachments has posts_id column that refers to id column in posts table (it doesn't say anything about how many attachments per post is allowed)

  • through relation is used in case of indirect relations, so for example if users has-many posts, and posts has-many attachments, we can make a call to find-all that will give us users with all the attachments of their posts.

Have in mind that unlike ActiveRecord, here associations are always plural (:posts {:authors :belongs-to} and not :posts {:author :belongs-to). The same applies to key names (users_id, not user_id). This is by design, and it's not likely to change.

Also, unlike 'ActiveRecord', here you can define through association referring to belongs-to association (the lack of this feature in 'ActiveRecord' is described for example here: https://www.ruby-forum.com/topic/74219)

Different name of an association than a table name

Sometimes you need to set an association that is named differently than the target table name, for example posts may have association authors which refers to table users (or another case: you need associations: created_by and updated_by, both referring to the same users table). In such case you can use inverse-of and model in associations hash, e.g.:

(def associations {:users {:posts {:type :has-many :inverse-of :authors}}
                   :posts {:authors {:type :belongs-to :model :users}}})

Unusual naming for keys/foreign keys

By default keys of tables are assumed to be called id and foreign keys are assumed to match the format association_id (so, for example foreign key for table users is called users_id). If you want to change that, you can define key patterns in last attribute of the function set-associations, e.g.

(def db-state (data-model/set-associations initial-db-state associations {
    :foreign-key-format #(str % "_key")}))

With above settings relational_mapper will expect foreign keys to match the pattern assocation_key.


relational-mapper uses Honey SQL for defining SQL conditions.


  • test it with MySQL (I used it only with PostgreSQL, it should work with MySQL too, but I haven't checked that),

  • improve performance (right now a database call is made per each requested table, while in some cases only one call could be made).

  • consider posibility of result having a deeper structure (result might be array of :posts with :authors and :attachments but then each attachment can have data from :files included)


Copyright © 2016 Krzysztof Herod

Distributed under the Eclipse Public License, the same as Clojure.