Datomic Knowledgebase - Recent questions tagged pull query

Why is the pull pattern for wild card giving me an error?

Wed, 31 Jan 2024 16:27:11 +0000

I am having an issue with a query and wild card patterns:
(d/q {:query '[:find (pull ?page pattern)

                         :in $ ?id pattern
                         :where
                         [?page :page/id ?id]]
                  :args [(d/db conn)
                         id
                         '[*]]})

It gives me an error:
Attribute identifier clojure.core/* of class: class java.lang.String does not start with a colon
Any idea why it doesn't work? I also tried adding the pattern directly to the pull expression, but I get the exact same issue

How to get io-stats from pull and pull-many?

Wed, 08 Mar 2023 15:04:12 +0000

I tried looking here but it doesn't seem to be documented https://docs.datomic.com/on-prem/clojure/index.html

I was able to get the io-stats from query, thanks to an example on this page: https://docs.datomic.com/on-prem/api/io-stats.html#query

Inconsistency between d/pull and d/q + pull for non-existent entities

Sat, 10 Dec 2022 15:55:29 +0000

Inconsistency between d/pull and d/q + pull for non-existent entities

Datomic returns a map with the :db/id of the entity being pulled, even if the entity doesn't exist in the database. For example:

(d/pull (d/db conn) '[*] 9999999999)
=> #:db{:id 9999999999}

This behavior is somewhat confusing, especially when using d/pull with time filters. I was expecting d/pull to return nil or #:db{:id nil} if the entity wasn't created at the given point in time. However, Datomic always returns a map with the entity ID:

(d/pull (as-of (d/db conn) 1) '[*] 9999999999)
=> #:db{:id 9999999999}

Surprisingly, I see the expected behavior if I pull the entity information from a query using d/q:

(d/q '{:find [(pull ?e [*])]
        :in   [?e]}
      (d/db conn) 9999999999)
=> [[#:db{:id nil}]]

Is that difference in behavior expected? How can I determine if an entity existed at any point in time using d/pull?