hi all. I’m having a bit of trouble with what I thought would be a simple create transaction, creating a post entity with an existing author:

(defn createPost [id to-be-created]
  (def conn (d/connect (:db-uri env)))
  (let [post-body (:body to-be-created)
        post-name (:name to-be-created)]
    (d/transact
          conn
          [[:db/add (d/tempid :db.part/user)
            :post/public-id id
            :post/body post-body
            :post/name post-name
            :post/author [:author/email “"]]])))

Is there something simple I’m missing? I’m new to datomic.

the :author/email is marked as identity

@davin: what's the error?

Yeah, that’s the strange thing. It isn’t throwing an error, but I don’t see a registered transaction in the console either

I’m running everything locally with bin scripts

d/transact returns a Future, you won't see an error unless you deref it.

@stuartsierra: ok I’ll pipe to deref and see what comes back, thanks

@davin: aren't you mixing list form and map form of transaction?

@yonatanel: I might be, I’m not familiar enough with it yet to know the difference 🙂

@davin: @yonatanel is correct. You look to be using the map form, but it’s in a list. You can replace the :db/add with :db/id and turn the whole thing into a map.

http://docs.datomic.com/transactions.html#transaction-structure

@davin: look here: http://docs.datomic.com/transactions.html#transaction-structure

thanks both of you, I’ll have a look at the link

You also want to be careful defining a connection inside a function that transacts

🙂

@marshall: I’m refreshing the conn everytime until I’m more familiar, what do you recommend?

Managing things like your connection is something people often use tools like Stuart Sierra’s Component for https://github.com/stuartsierra/component

In general the connection is something shared across your program, so defining it in a single function is not ideal. Functions that transact data should probably take a connection as an argument, but also keep in mind you’ll want to be careful about that if all you’re doing is reading (http://www.rkn.io/2014/02/10/datomic-antipatterns-connnnn/)

Thanks @yonatanel @stuartsierra and @marshall, it was the list vs map form that was the problem. 🙂

Thanks @marshall I’ll read everything you linked to

Also, don't def inside another def — def is always global, regardless of where it appears.

Yes, the def inside did seem weird, I probably meant it to be a let

Will the conn timeout at some point?

if it is defined more globally?

Is there a datomic cookbook or something similar except the best practices section of the docs, with explanation of each solution and addressing performance, indexes, gotchas etc?

Is it possible to write a retraction query, like 'retract all entities that have a :from attribute 1'?

and/or if I have a schema like:

(def schema
  {:to {:db/type :db.type/ref}
   :from {:db/type :db.type/ref}})

when I [:db.fn/retractEntity 1]

(I'm using DataScript, so maybe it is just a missing feature, hmmm I shall try it in Datomic proper)

@timothypratley: You might want to look at the retractEntity database function: http://docs.datomic.com/transactions.html#built-in-transaction-functions

@marshall right, rectractEntity appears to only work with a provided id or lookup ref, what I'm wondering is whether I need to do 2 steps: a) lookup all the entities, b) retract them... or if there is some way to in a single transaction identify and retract them together.

In this case I'm representing nodes and edges. When deleting a node, some edges become invalid and need to be removed. So I find the edges and remove them. But it seems like it should be possible to do it in a transaction, instead of as a lookup + transaction.

retractEntity will remove all references from and to a given entity it does this by retracting all datoms with that entity ID in either the e or v position if you need to retract more than one entity this way, then yes, you’ll need to create a list of them (presumably via query) and call retractEntity on each

if you want it all to occur in a single transaction, you can write a transaction function that does the lookup and calls retractEntity on all results of the lookup

ah so in the case above it removes the :from, but leaves the :to (which might point to 2) -- which is an invalid edge -- so I need to go with writing a transaction function, thanks.

I am getting java.lang.NoClassDefFoundError: Could not initialize class datomic.ddb_cluster__init when calling (d/create-database "datomic:). Is there something else I need to do besides adding com.amazonaws/aws-java-sdk-dynamodb to my project?

I am running the transactor locally and I am running create-database in a REPL.

Also, I'm using datomic 0.9.5385

Also probably relevant: I can create a test database using Datomic shell

Hmm.. After restarting the REPL, the first time I run create-database I get java.lang.ClassNotFoundException: com.amazonaws.DnsResolver but every following time I run create-database I get the previous error, java.lang.NoClassDefFoundError: Could not initialize class datomic.ddb_cluster__init.

Hmm.. just updated to 1.11.22 and it worked. You guys should update the docs.

Hi all, how possible is it to run Datomic on DB2?