Datomic - joins between entities where there is no reference

Question

My question is - is there a way to join two entities in Dataomic, that don't have a reference in the schema, without resorting to writing two nested iterators (a manual join).

In the Datomic Doco - they give an example of a query specifying two parameters.

[:find ?n ?u
 :where
 [?c :community/name ?n]
 [?c :community/url ?u]]

They call this a 'join' - because the underlying structure is a key-value database - so even attributes of the same entity need to be 'joined' together.

Then they give an example of a join between two entities that have a reference (assume the reference is defined in the schema which is not shown here):

[:find ?c_name
 :where
 [?c :community/name ?c_name]
 [?c :community/neighborhood ?n]
 [?n :neighborhood/district ?d]
 [?d :district/region :region/ne]]

My question is - is a query like the above possible without a reference in the schema? Or would I have to resort to writing an iterator and trawling through the results?

Answer 1

Any variable that occurs more than once in the :where clause is implictly joined over.

So you can ask for names that belong to a community as well as to a neighbourhoord

(def results (q '[:find ?name :where [_ :neighborhood/name ?name] 
                                     [_ :district/name ?name]] 
                 (db conn)))

Is this what you were asking for? (ids are just ignored in this example)