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‎databases/databases.asciidoc‎

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include::traversing-datomic-indices/traversing-datomic-indices.asciidoc[]
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include::datomic/traversing-indices/traversing-indices.asciidoc[]

‎databases/datomic/traversing-indices/traversing-indices.asciidoc‎

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=== Traversing Datomic Indices
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[role="byline"]
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by Alan Busby and Ryan Neufeld
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==== Problem
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You want to execute simple Datomic queries with high-performance.
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==== Solution
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Use the +datomic.api/datoms+ function to directly access the core
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Datomic indices in your database.
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To follow along with this recipe, complete the steps in
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<<sec_datomic_connect_to_datomic_solution>> and
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<<sec_datomic_schema_solution>>. After doing this you will have a
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connection, +conn+, and a schema installed against which you can
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insert data.
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For example, to quickly find the entities which have the provided attribute and
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value set, invoke +datomic.api/datoms+, specifying the +:avet+ index
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(attribute, value, entity, transaction) and the desired attribute and
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value.
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[NOTE]
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====
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This will only work for attributes where +:db/index+ is true, or
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+:db/unique+ is not nil.
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====
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[source,clojure]
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----
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(require '[datomic.api :as d])
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(d/transact conn [{:db/id (d/tempid :db.part/user)
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:user/name "Barney Rubble"
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:user/email "[email protected]"}])
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(defn entities-with-attr-val
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"Return entities with a given attribute and value."
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[db attr val]
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(->> (d/datoms db :avet attr val)
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(map :e)
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(map (partial d/entity db))))
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(def barney (first (entities-with-attr-val (d/db conn)
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:user/email
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"[email protected]")))
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(:user/email barney)
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;; -> "[email protected]"
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----
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To quickly determine all of the attributes an entity has, use the
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+:eavt+-ordered index.
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[source,clojure]
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----
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(defn entities-attrs
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"Return attrs of an entity"
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[db entity]
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(->> (d/datoms db :eavt (:db/id entity))
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(map :a)
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(map (partial d/entity db))
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(map :db/ident)))
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(entities-attrs (d/db conn) barney)
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;; -> (:user/email :user/name)
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----
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To quickly find entities that refer, via +:db.type/ref+, to a provided
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entity, use the +:vaet+-ordered index.
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[source,clojure]
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----
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;; Add a person that refers to a :user.roles/author role
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(d/transact conn [{:db/id (d/tempid :db.part/user)
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:user/name "Ryan Neufeld"
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:user/email "[email protected]"
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:user/roles [:user.roles/author :user.roles/editor]}])
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(defn referring-to
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"Find all entities referring to an entity as a certain attribute."
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[db entity]
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(->> (d/datoms db :vaet (:db/id entity) )
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(map :e)
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(map (partial d/entity db))))
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(def author-entity (d/entity (d/db conn) :user.roles/author))
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;; The names of all users with a :user.roles/author role
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(map :user/name (referring-to (d/db conn) author-entity))
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;; -> ("Ryan Neufeld")
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----
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==== Discussion
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For simple lookup queries, like "find by attribute" or "find by
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value", nothing beats Datomic's raw indices in terms of performance.
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The +datomic.api/datoms+ interface provides access to all of Datomic's
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indices, and conveniently lets you dive in any number of levels,
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"biting off" only the data you need.
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As with most Datomic functions, +datoms+ takes a +db+ as its first
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argument. You'll not in our examples, and elsewhere in the book, we
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too accept a database as a value, and not a connection--this idiom
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allows API users to perform varying numbers of operations on the same
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database value. You should always try to do this yourself.
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+datoms+ second argument is the particular index you want to access.
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Each of these is a permutation of e, entity, a, attribute, v, value,
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and t, transaction. The order of letters in an index indicates how it
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is indexed. For example, +:eavt+ should be traversed by entity, then
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attribute, then so on and so forth. The four indices and what they
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include are as follows:
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* +:eavt+ - An entity-first index which includes all datoms. This
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index provides a view over your database very much like a
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traditional relational-database.
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* +:aevt+ - An "attribute, then entity" index which includes all datoms. This
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index provides columnar access to your database, much like a data
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warehouse.
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* +:avet+ - An "attribute-value" index which only includes attributes
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where +:db/index+ is +true+. Incredibly useful as a lookup index
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(e.g. "I need the entity with email of '[email protected]'").
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* +:vaet+ - A value-first index which only includes +:db.type/ref+
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values. A very interesting index that can be used to treat your data
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a bit like a graph database.
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After specifying an index ordering, you can optionally provide any
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number of components to pre-traverse the index. This serves to reduce
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the number of elements returned. For example, specifying just an
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attribute component for AVET traversal will return any entity with
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that attribute. Specifying an attribute and a value component, on the
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other hand, will return only entities with that specify attribute and
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value pair.
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What is returned by +datoms+ is a stream of +Datum+ objects. Each
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datum responds to +:a+, +:e+ and +:v+ as functions, returning the
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attribute, entity and value of the datom, respectively.

‎databases/traversing-datomic-indices/traversing-datomic-indices.asciidoc‎

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