Schema transaction

To create our Datomic database we need to transact some schema transaction data in Datomic. This makes our defined attributes available in Datomic.

Schema transaction data

Molecule transforms our Schema definition file to basically a java.util.List containing a java.util.Map of schema transaction data for each attribute defined. Our name and url attributes for instance requires the following map of information to be transacted in Datomic:

object SeattleSchema extends Transaction {
  lazy val partitions = Util.list()

  lazy val namespaces = Util.list(
    // Community --------------------------------------------------------":db/ident"             , ":community/name",
             ":db/valueType"         , ":db.type/string",
             ":db/cardinality"       , ":db.cardinality/one",
             ":db/fulltext"          , true.asInstanceOf[Object],
             ":db/index"             , true.asInstanceOf[Object],
             ":db/id"                , Peer.tempid(":db.part/db"),
             ":db.install/_attribute", ":db.part/db"),":db/ident"             , ":community/url",
             ":db/valueType"         , ":db.type/string",
             ":db/cardinality"       , ":db.cardinality/one",
             ":db/index"             , true.asInstanceOf[Object],
             ":db/id"                , Peer.tempid(":db.part/db"),
             ":db.install/_attribute", ":db.part/db"),
    // etc...

As you see, the Community namespace information is present in the value of the first pair in the map for the name attribute:

":db/ident", ":community/name",

The rest of the lines are pretty self-describing except from the last two that create and save the internal id of the attribute in Datomic. Datomic schemas are literally a set of datoms that have been transacted as any other data!

Partition transaction data

Partition transaction data looks almost like attribute transaction data:

lazy val partitions = Util.list(":db/ident"             , ":gen",
         ":db/id"                , Peer.tempid(":db.part/db"),
         ":db.install/_partition", ":db.part/db"),

… except that the information is now installed internally in Datomic as partition data instead of as attribute data.

Partition examples with Molecule:

More about partitions in Datomic.

Create new Datomic database

Now we can simply pass the generated raw transaction data to Datomic in order to create our partitions/schema:

implicit val conn = recreateDbFrom(SeattleSchema)

The returned connection to the database is saved in an implicit val. Molecule method calls need an implicit database connection to be in scope so our above implicit conn object will make it possible to create and operate on molecules in the following code.

Managing databases

Datomic databases are created with a database name so that we can later refer to a spedific database. In the above creation example, a random database name was created which is convenient for testing purposes.

For durable databases we use a database name:

// Create new database with identifier
implicit val conn = recreateDbFrom(SeattleSchema, "myDatabase")

Then we can later - in another scope - establish a new connection to the existing database:

// Create connection to the database 'myDatabase' 
implicit val conn = Conn("myDatabase")


We can also supply a protocol like ‘mem’ for in-memory db, or ‘dev’ for a development db saved on local disk etc.

// Create new database with identifier as an in-memory database
implicit val conn = recreateDbFrom(SeattleSchema, "myDatabase", "mem")

For more information on db creation, please see the Datomic documentation.


Since Molecule is in its early development (although started in 2014) a more thorough Molecule database management API hasn’t been created yet. It is though no problem to access Datomic directly for this purpose in the meanwhile. The recreateDbFrom method is for instance just a wrapper around the following code:

import datomic._

// Setup database
val uri = "datomic:mem://seattle"
implicit val conn = Conn(Peer.connect(uri))

// Transact partitions/schema
conn.transact(SeattleSchema.partitions) // Optional

So if you need more functionality than currently provided in Molecule you can simply use the datomic API and likely methods of the Peer. Or you can file an issue or send a pull request :-)