Get (read) Data

We get/read data from the database by calling get on a molecule. This returns a List of tuples that match the molecule attributes (except for arity-1):

val persons1attr: List[String] =

val persons2attrs: List[(String, Int)] =

val persons3attrs: List[(String, Int, String)] =

// Etc.. to arity 22

Data can be returned in 5 different formats:

// List for convenient access to smaller data sets
val list : List[(String, Int)] = m(

// Mutable Array for fastest retrieval and traversing
val array: Array[(String, Int)] = m(

// Iterable for lazy traversing with an Iterator
val iterable: Iterable[(String, Int)] = m(

// Json formatted string 
val json: String = m(

// Raw untyped Datomic data if data doesn't need to be typed
val raw: java.util.Collection[java.util.List[AnyRef]] = m(

Async API

Molecule provide all operations both synchronously and asynchronously, so the 5 getter methods also has equivalent asynchronous methods returning data in a Future:

val list    : Future[List[(String, Int)]] = m(
val array   : Future[Array[(String, Int)]] = m(
val iterable: Future[Iterable[(String, Int)]] = m(
val json    : Future[String] = m(
val raw     : Future[java.util.Collection[java.util.List[AnyRef]]] = m(

With entity id

Attributes of some entity are easily fetched by applying an entity id to the first namespace in the molecule

Person(fredId).name.age.likes.get.head === List("Fred", 38, "pizza")

The entity id is used for the first attribute of the molecule, here name having entity id fredId.

Person is just the namespace for the following attributes, so that we get :Person/name, :Person/age, :Person/likes etc..

2-steps with Entity API

Molecules can get optional attributes which make them flexible to get irregular data sets. We could for instance fetch some Persons where some of them has no likes asserted:$.get === List(
  ("Fred", 38, Some("pizza")),
  ("Lisa", 7, None),
  ("Ben", 5, Some("pizza"))

An alternative is to get the data in 2 steps:

  1. Define the shape of the data set with a molecule and get the entity ids
  2. Get attribute values using the entity api

This way we can for instance in the first step get the mandatory data (name and age) with a molecule and then in a second step ask for optional data (likes) for each entity:

// Step 1
val seed: List[(Long, String, Int)] =

// Step 2
val data: List[(String, Int, Option[String])] = { case (e, name, age) =>
  // Add optional `likes` value via entity api
  (name, age, e[String](":Person/likes"))

For this simple example, the original molecule with an optional likes attribute would of course have been sufficient and more concise. But for more complex interconnected data this approach can be a good extra tool in the toolbox.


Get Json…