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MapGraph

A basic in-memory database for storing linked maps in Clojure and ClojureScript

Releases and Dependency Information

Leiningen dependency information:

[com.stuartsierra/mapgraph "0.2.1"]

Maven dependency information:

<dependency>
  <groupId>com.stuartsierra</groupId>
  <artifactId>mapgraph</artifactId>
  <version>0.2.1</version>
</dependency>

Gradle dependency information:

compile "com.stuartsierra:mapgraph:0.2.1"

Dependencies and Compatibility

MapGraph is written in .cljc and depends on Clojure or ClojureScript version 1.7.0 or higher.

To run the tests you will need clojure.spec, available in Clojure 1.9.0-alpha5 or higher.

Discussion

Please post questions on the Clojure Mailing List

Usage

(ns examples
  (:require [com.stuartsierra.mapgraph :as mg]))

Create a new MapGraph database with new-db. You will probably want to store it in a mutable reference such as an Atom.

(def db (atom (mg/new-db)))

Add the unique identity attributes that define your schema.

(swap! db mg/add-id-attr :user/id :color/hex)

Add entities to your database with add. You can add multiple entities at once, and they may be nested.

(swap! db mg/add
       {:user/id 1
        :user/name "Pat"
        :user/favorite-color {:color/hex "9C27B0"
                              :color/name "Purple"}}
                          ;  ^-- nested entity

       {:user/id 2
        :user/name "Reese"
        :user/favorite-color {:color/hex "D50000"
                              :color/name "Red"}})

Entities in the database are stored normalized: all nested entities are replaced with lookup refs. You can see this if you get an entity by its lookup ref.

(get @db [:user/id 2])
;;=> {:user/id 2,
;;    :user/name "Reese",
;;    :user/favorite-color [:color/hex "D50000"]}
                        ;  ^-- lookup ref

To get back nested entities, use pull, which takes a pattern describing which attributes and entities you want to get back. It is similar to Datomic Pull.

(mg/pull @db
         [:user/name {:user/favorite-color [:color/name]}]
         [:user/id 2])
;;=> {:user/name "Reese",
;;    :user/favorite-color {:color/name "Red"}}

Entities with the same unique identity are merged.

(swap! db
       mg/add
       {:user/id 1  ; "Pat"
        :user/profession "Programmer"})

(mg/pull @db
         [:user/id :user/name :user/profession]
         [:user/id 1])
;; {:user/id 1,
;;  :user/name "Pat",
;;  :user/profession "Programmer"}

Entities can refer to other entities, forming a graph. The graph may have cycles.

(swap! db
       mg/add
       {:user/id 1
        :user/friends #{{:user/id 2}}}
       {:user/id 2
        :user/friends #{{:user/id 1}}})

(mg/pull @db
         [:user/name
          {:user/friends [:user/name
                          {:user/friends [:user/name]}]}]
         [:user/id 1])
;;=> {:user/name "Pat",
;;    :user/friends #{{:user/name "Reese",
;;                     :user/friends #{{:user/name "Pat"}}}}}

To remove an entity, dissoc its lookup ref. Dangling lookup refs will be ignored on subsequent pull.

(swap! db dissoc [:user/id 2])  ; Reese

(mg/pull @db '[*] [:user/id 2])
;;=> nil

(mg/pull @db
         [:user/name
          {:user/friends [:user/name]}]
         [:user/id 1])
;;=> {:user/name "Pat",
;;    :user/friends #{}}
                  ;  ^-- Reese is gone

Collections

Attribute values can be any Clojure collection type.

(swap! db mg/add
       {:user/id 1
        :user/favorite-sports '(hockey tennis golf)})

(mg/pull @db
         [:user/name :user/favorite-sports]
         [:user/id 1])
;;=> {:user/name "Pat", :user/favorite-sports (hockey tennis golf)}

Merging a new collection value completely replaces the previous value.

(swap! db mg/add
       {:user/id 1
        :user/favorite-sports '(tennis polo)})

(mg/pull @db
         [:user/name :user/favorite-sports]
         [:user/id 1])
;;=> {:user/name "Pat", :user/favorite-sports (tennis polo)}

A collection of nested entities may be a list, vector, set, or map in which the vals are entities.

(def sample-host
  {;; identifier
   :host/ip "10.10.1.1"

   ;; non-entity value
   :host/name "web1"

   ;; collections (list, vector, set, map) of non-entity values
   :host/aliases ["host1" "www"]
   :host/rules {"input" {"block" "*", "allow" 80}
                "output" {"allow" 80}}

   ;; single entity value
   :host/gateway {:host/ip "10.10.10.1"}

   ;; collection of entities (list, vector, set)
   :host/peers #{{:host/ip "10.10.1.2", :host/name "web2"}
                 {:host/ip "10.10.1.3"}}

   ;; map of non-entity keys to entity vals
   :host/connections {"database"         {:host/ip "10.10.1.4", :host/name "db"}
                      ["cache" "level2"] {:host/ip "10.10.1.5", :host/name "cache"}}})

pull works the same way on single entities and collections of entities.

(def hosts
  (atom (-> (mg/new-db)
            (mg/add-id-attr :host/ip)
            (mg/add sample-host))))

(mg/pull @hosts
         [:host/ip
          :host/rules
          {:host/gateway [:host/ip]
           :host/peers [:host/ip]
           :host/connections [:host/name]}]
         [:host/ip "10.10.1.1"])
;;=> {:host/ip "10.10.1.1",
;;    :host/rules {"input" {"block" "*", "allow" 80},
;;                 "output" {"allow" 80}},
;;    :host/gateway {:host/ip "10.10.10.1"},
;;    :host/peers #{{:host/ip "10.10.1.3"}
;;                  {:host/ip "10.10.1.2"}},
;;    :host/connections {"database" {:host/name "db"},
;;                       ["cache" "level2"] {:host/name "cache"}}}

Collections may not mix entities and non-entities.

(try (swap! db mg/add {:user/id 3 :user/friends [{:user/id 1} "Bob"]})
     (catch Throwable t t))
;; #error {:data {:reason ::mg/mixed-collection,
;;                ::mg/attribute :user/friends,
;;                ::mg/value [{:user/id 1} "Bob"]}}

Comparison with Datomic/Datascript

MapGraph is designed to be used as a temporary store for data kept in Datomic or Datascript.

MapGraph is different from Datomic/Datascript in the following ways:

  • Schema only specifies unique identity attributes

  • Non-identity attributes do not need to be declared before they are used

  • An entity must not have more than one unique identity attribute

  • Values may include collections of any type

  • Empty collections will be stored rather than ignored

  • Updating the value of an attribute with a collection always replaces the entire previous value

  • No reverse attribute references (like :user/_friends)

  • No component attributes

  • pull does not support recursion, default values, limits, or reverse lookup

  • No indexes

  • No queries, only lookup by unique identity attribute

  • No database entity IDs, only lookup refs

Bug reports

Please file issues on GitHub with minimal sample code that demonstrates the problem.

Contributing

Please do not send pull requests without prior discussion. Please contact me via email first. Thank you.

Special thanks to

Jeb Beich for discussion, early testing, and contributions.

Cognitect for providing me with time to work on open-source projects. This library is my personal work and is not officially supported by Cognitect, Inc.

Copyright and License

The MIT License (MIT)

Copyright (c) 2016 Stuart Sierra

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.