SubGraph

Reactive graph database for re-frame

Releases and Dependency Information

• SNAPSHOT only, stable release coming soon

• All releases

Leiningen dependency information:

[vimsical/subgraph "0.1.0-SNAPSHOT"]

Maven dependency information:

<dependency>
  <groupId>vimsical</groupId>
  <artifactId>subgraph</artifactId>
  <version>0.1.0-SNAPSHOT</version>
</dependency>

Gradle dependency information:

compile "vimsical:subgraph:0.1.0-SNAPSHOT"

Dependencies and Compatibility

SubGraph is written in .cljc and depends on Clojure or ClojureScript version 1.8.0 or higher.

re-frame and reagent are provided dependencies and will need to be included in your own project's dependencies.

Terminology

Entity

SubGraph entities are regular maps that can be uniquely identified by one (and currently only one) of their entries.

Lookup ref

A lookup-ref is a 2-element vector representing an entity's identifying key and its associated value.

Normalization

Normalization is the process of eliminating redundancy in entities by replacing every the other entities that it references, aka joins, with lookup refs.

Database

The SubGraph database is a regular map associating lookup refs to their corresponding entities.

Usage

This section shows how to initialize a SubGraph database, add, query and update entities using re-frame event handlers and subscriptions.

For more usage examples of interacting directly with the database, refer to the usage section in the MapGraph README

In the following examples we'll setup a reactive database dealing with users and their favorite colors. When denormalized the data looks like this:

  {:user/id             1
   :user/name           "Pat"
   :user/favorite-color {:color/hex  "9C27B0"
                         :color/name "Purple"}
   :user/friends        [{:user/id             2
                          :user/name           "Reese"
                          :user/favorite-color {:color/hex  "D50000"
                                                :color/name "Red"}}]}

To get started we'll need re-frame, subgraph and the interop namespaces.

(ns example
  (:require
   [re-frame.core :as re-frame]
   [vimsical.subgraph :as sg]
   [vimsical.subgraph.re-frame :as sg.re-frame]))

Database initialization

In order to avoid duplication we want to normalize not only users, but colors as well, looking at our denormalized data we see that our identifying attributes are :user/id and :color/hex.

We can create a new empty database with sg/new-db but for normalization to work we'll have to configure it by adding our identifying attributes using add-id-attr.

(defn new-db
  []
  (-> (sg/new-db)
      (sg/add-id-attr :user/id :color/hex)))

We're now able to produce a new empty db value, so we register a re-frame handler to initialize our app db, and dispatch it right away.

(re-frame/reg-event-db ::new-db (constantly (new-db)))

(re-frame/dispatch [::new-db])

Adding entities

We can populate our database using add which accepts a variable number of entities. In order to be able to add data in reaction to user input in a component we'll need to invoke that function inside a re-frame event handler. The most generic of such handlers simply wraps add.

(re-frame/reg-event-db
 ::add
 (fn [db [_ & entities]]
   (apply sg/add db entities)))

We can now dispatch ::add events to populate our database. Note that nested entities are valid and will normalize recursively according to the attributes added with add-id-attr.

(re-frame/dispatch
 [::add
  {:user/id             1
   :user/name           "Pat"
   :user/favorite-color {:color/hex  "9C27B0"
                         :color/name "Purple"}}
  {:user/id             2
   :user/name           "Reese"
   :user/favorite-color {:color/hex  "D50000"
                         :color/name "Red"}}])

Subscriptions and pull queries

One of the design goals of SubGraph was to enable fully reactive pull queries against a (r)atom. Reactive pull queries return reactions that not only update with changes in that entity's pattern, but recursively for any joined entity.

vimsical.subgraph.re-frame/pull is an api-compatible version of vimsical.subgraph/pull designed to work with (r)atoms. For convenience we also provide a re-frame raw subscription handler vimsical.subgraph.re-frame/raw-sub-handler.

We register a generic subscription handler that we'll call :q.

(re-frame/reg-sub-raw :q sg.re-frame/raw-sub-handler)

This subscription accepts any pattern and lookup-ref, and will return a fully reactive reaction graph.

(deref
 (re-frame/subscribe
  [:q 
   ;; Pattern
   [:user/name {:user/favorite-color [:color/name]}]
   ;; Lookup ref
   [:user/id 2]]))

;; => #:user{:name "Reese", :favorite-color #:color{:name "Red"}}

Updates

To update an entity we simply add it again to the database, the semantics are equivalent to that of merge.

Let's create a cycle by updating Pat and Reese, making them friends by referencing each other.

(re-frame/dispatch
 [::add
  {:user/id 1 :user/friends #{{:user/id 2}}}
  {:user/id 2 :user/friends #{{:user/id 1}}}])

Since queries can be arbitrarily nested, we can ask for Pat's friends' favorite colors.

(deref
 (re-frame/subscribe
  [:q
   [:user/name
    {:user/friends
     [:user/name {:user/favorite-color [:color/name]}]}]
   [:user/id 1]]))

;; => #:user{:name "Pat", :friends #{#:user{:name "Reese", :favorite-color #:color{:name "Red"}}}}

And then update Reese's favorite color.

(re-frame/dispatch
 [::add
  {:user/id             2
   :user/favorite-color {:color/hex  "1789d6"
                         :color/name "DodgerBlue3"}}])

Thanks to normalization and our reactive graph, our subscription for Pat's friends' favorite colors updates, showing Reese's new favorite color.

(deref
 (re-frame/subscribe
  [:q
   [:user/name
    {:user/friends
     [:user/name {:user/favorite-color [:color/name]}]}]
   [:user/id 1]]))

;; => #:user{:name "Pat", :friends #{#:user{:name "Reese", :favorite-color #:color{:name "DodgerBlue3"}}}}

Comparison with MapGraph

SubGraph builds on a fork of Stuart Sierra's MapGraph, the implementation diverged in order to add support for (r)atoms in the pull api.

The vimsical.subgraph namespace is api-compatible with com.stuartsierra.mapgraph, however SubGraph extends the pull query syntax with support for:

• Recursive join queries

Unbounded recursion should be used with caution since there is no mechanism to detect cycles. In our example of mutual friends, the following query would run infinitely .

[:user/name
 {:user/favorite-color [:color/name]}
 {:user/friends '...}]

A number can be provided to limit the level of nesting.

[:user/name
 {:user/favorite-color [:color/name]}
 {:user/friends 3}]

• Link references

Applications commonly need to keep track of global references, such as the current user or a selection. This is easily achieved by storing a lookup-ref as a value in the database, for example {:app/user [:user/id 1]}.

SubGraph supports this pattern with a special query syntax identical to that of om.next.

[{[:app/user '_] 
  [:user/name
    {:user/favorite-color [:color/name]}
    {:user/friends 3}]}]

Comparison with om.next

Limitations

• SubGraph currently doesn't support union queries
• SubGraph's query parser is not extensible, as such there is no support for parametrized joins and mutations

Differences

• SubGraph doesn't have an indexer and relies on reagent reactions to update components when data changes.
• Normalization is driven by the database's id-attrs, when adding entities no query, or component tree, is required.

Comparison with Datomic/Datascript

Refer to the comparison in the MapGraph Repo

Bug reports

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

Contributing

Pull requests are welcome!

Special thanks to

Stuart Sierra for MapGraph

David Nolen for ClojureScript and om.next