3 attribute modes

1. Mandatory attr

When we use a molecule to query the Datomic database we ask for entities having all our Attributes associated with them.

Note that this is different from selecting rows from a sql table where you can also get null values back!

If for instance we have entities representing Persons in our data set that haven’t got any age Attribute associated with them then this query will not return those entities:

val persons = Person.name.age.get

Basically we look for matches to our molecule data structure.

2. Tacet attr_

Sometimes we want to grap entities that we know have certain attributes, but without returning those values. We call the un-returning attributes “tacit attributes”.

If for instance we wanted to find all names of Persons that have an age attribute set but we don’t need to return those age values, then we can add an underscore _ after the age Attribute:

val names = Person.name.age_.get

This will return names of person entities having both a name and age Attribute set. Note how the age values are no longer returned from the type signatures:

val persons: Iterable[(String, Int)] = Person.name.age.get
val names  : Iterable[String]        = Person.name.age_.get

This way we can switch on and off individual attributes from the result set without affecting the data structures we look for.

3. Optional attr$


If an attribute value is only sometimes set, we can ask for it’s optional value by adding a dollar sign $ after the attribute:

val names: Iterable[(String, Option[String], String)] = Person.firstName.middleName$.lastName.get

That way we can get all person names with or without middleNames. As you can see from the return type, the middle name is wrapped in an Option.


Map attributes…