Datomic Pitfalls

by Christian Westrom

What is Datomic?

Datomic is a database. From what I hear, it's much nicer to work with than SQL. I don't want to worry about concatenating strings to make queries. I just want to pass around Clojure data like I always do.

Anyway, I went along the documentation trying to figure out how to use Datomic, and I just couldn't get passed a couple blocks until a friend from lambdaisland helped me out.

The two pitfalls of learning Datomic.

Where do I get Datomic in the first place?

This isn't really a pitfall, so much as it was not easy for me to find this information. So when I started out, I used a library called Datalevin, which mimics Datomic's API. It was good enough for my purposes, but it isn't quite up to par with Datomic itself. Anyway, the free version is called com.Datomic/Datomic-free. You can add the latest version to your project by copying the info from clojars.

Pitfall #1 - Schema-Data Mismatch

A schema looks like a vector of maps. Each map will have 3 required attributes.

These attributes are:

  1. :db/ident

  2. :db/valueType

  3. :db/cardinality

These get passed into a map. Looking something like this:

{:db/ident       :item/id-number
 :db/valueType   :db.type/int
 :db/cardinality :db.cardinality/one}

You will get an error if you try to pass in data that doesn't match the type you defined. I encountered this when parsing csv data as the csv library I was using takes everything as strings.

{:item/id-number (Integer/parseInt "1111")}

Pitfall #2 - Query mistakes

Queries are written in a language called datalog, a variant of prolog for data.

Querying the connection won't work.

(d/q '[:find ?e
       :where [?e :item/item-number]] conn)

For some reason it's abstracted by the `db` function.

(d/q '[:find ?e
       :where [?e :item/item-number]] (d/db conn))

This one won't work.

(d/q [:find ?e
      :where [?e :item/item-number]] (d/db conn))

The query must be quoted before passed to the query function.

(d/q '[:find ?e
       :where [?e :item/item-number]] (d/db conn))

The one above returns the IDs of each entity.

This one reuturns the items.

(d/q '[:find ?c
       [?e :item/item-number ?c]] (d/db conn))