Schema Data Reference

'' literal
"" string
[] = list or vector
{} = map {k1 v1 ...}
() grouping
| choice
? zero or one
+ one or more

attr-def       = {':db/ident' keyword
                  ':db/cardinality' cardinality
                  ':db/valueType' type
                  (':db/doc' string)?
                  (':db/unique' unique)?
                  (':db/isComponent' boolean)?
                  (':db/id' n-identifier)?
                  (':db/noHistory' boolean)?
                  (':db.attr/preds' attr-pred+)?
                  (':db.entity/attrs' ident+)?
                  (':db.entity/preds' ent-pred+)?}
n-identifier   =  (identifier | tempid)
identifier     = (eid | lookup-ref | ident)
eid            = nat-int
lookup-ref     = [identifier value]
ident          = keyword
attr-pred      = fully-qualified symbol that name a /predicate/ of a value
ent-pred       = fully-qualified symbol that name a /predicate/ of 
                 a database value and an entity id
cardinality    = (':db.cardinality/one' | ':db.cardinality/many)                   
type           = (':db.type/bigdec' | ':db.type/bigint' | ':db.type/boolean' | 
                  ':db.type/double' | ':db.type/float' | ':db.type/instant' | 
                  ':db.type/keyword' | ':db.type/long' | ':db.type/ref' | 
                  ':db.type/string' | ':db.type/symbol' | ':db.type/tuple' | 
                  ':db.type/uuid' | ':db.type/uri' )
unique         = (':db.unique/identity' | ':db.unique/value')
boolean        = ('true' | 'false')

The grammar above shows only the attributes that are built-in to Datomic schema. Schema entities can also have attributes that you define.

The grammar shows only the transaction map form. It is also possible to define schema with the more verbose transaction list form, as these forms are semantically equivalent.

The :db namespace, and all :db.* namespaces, are reserved for use by Datomic. With the exception of :db/doc, and :db/ident, you should not use the built-in Datomic attributes on your own domain entities.

Because schema is composed of ordinary Datomic data, the schema grammar is a specialization of the transaction grammar. The shared grammar elements n-identifier, identifier, eid, lookup-ref, and ident are documented there.

The allowable value of :db/ident is a namespaced keyword with the form :<namespace>/<name>. It is possible to define a name without a namespace, as in :<name>, but a namespace is preferred in order to avoid naming collisions. Namespaces can be hierarchical, with segments separated by ".", as in :<namespace>.<nested-namespace>/<name>.

The :db namespace, and all :db.* namespaces, are reserved for use by Datomic. With the exception of :db/doc and :db/ident, you should not use the built-in Datomic attributes on your own domain entities.

Use only letters, numbers, hyphens, and underscores in :db/ident values.

Specifies a boolean value indicating whether past values of an attribute should not be retained. Defaults to false. The purpose of :db/noHistory is to conserve storage, not to make semantic guarantees about removing information. The effect of :db/noHistory happens in the background, and some amount of history may be visible even for attributes with :db/noHistory set to true.

db/noHistory is often used for high churn attributes along with attributes that you do not require a history of.

Unique identity is specified through an attribute with db.unique set to db.unique/identity. Values of the attribute must be unique to each entity and upsert is enabled; attempts to insert a duplicate value for a temporary entity id will cause all attributes associated with that temporary id to be merged with the entity already in the database.

The tutorial creates an :inv/sku that is unique:

{:db/ident :inv/sku
 :db/valueType :db.type/string
 :db/unique :db.unique/identity
 :db/cardinality :db.cardinality/one}

Unique identity is appropriate whenever you want to assert a database-wide unique identifier for an entity. Common examples include emails, account names, product codes/skus, and UUIDs.

If a transaction specifies a unique identity for a temporary id, and that unique identity already exists in the database, then that temporary id will resolve to the existing entity in the system. This upsert behavior makes it possible for transactions to work with domain identities, without ever having to specify Datomic entity ids.

It is legal for a single entity to have multiple different unique attributes, e.g. you might decide that people have both unique emails and unique government identifiers. However, note that this creates the possibility of conflict, which will result in a conflict anomaly. (Conflict will occur if a transaction tries to upsert a tempid into two different existing entities. As an example. if entity 42 has the unique email"johndoe@example.com", and entity 43 has the unique account number 1007, then a transaction cannot claim that a new tempid is both John Doe and account 1007.)

Uniqueness can be declared on attributes of any value type, including references (:db.type/ref). Only (:db.cardinality/one) attributes can be declared unique.

Datomic does provide a mechanism to declare composite uniqueness constraints in Composite Tuples; however, you can also implement them (or any arbitrary functional constraint) via transaction functions.

Unique value is specified through an attribute with db/unique set to db.unique/value. A unique value represents an attribute-wide value that can be asserted only once.

Unique values have the same semantics as unique identities, with one critical difference: Attempts to assert a new tempid with a unique value already in the database will cause a conflict anomaly.

• Keywords are interned for efficiency.
• Instances are stored as the number of milliseconds since the epoch.
• Strings are limited to 4096 characters.
• BigDecimals are limited to 1024 digit precision.
• BigIntegers are limited to a bit length of 8192.
• Symbols map to the symbol type in languages that support them, e.g. clojure.lang.Symbol in Clojure
• Consistent results in query depend on the scale matching for all BigDecimal comparisons. You are strongly encouraged to use a consistent scale per attribute.

Tuples can be used to create multi-attribute unique keys on domain entities. Tuples can be used to optimize queries that otherwise would have to join two or more high-population attributes.

Note: Tuples require 480-8770 or later and a compatible base schema.

A tuple is a collection of 2-8 scalar values, represented in memory as a Clojure vector. There are three kinds of tuples:

• Composite tuples are derived from other attributes of the same entity. Composite tuple types have a :db/tupleAttrs attribute, whose value is 2-8 keywords naming other attributes.
• Heterogeneous fixed length tuples have a :db/tupleTypes attribute, whose value is a vector of 2-8 scalar value types.
• Homogeneous variable length tuples have a :db/tupleType attribute, whose value is a keyword naming a scalar value type.

The following types are considered scalar types suitable for use in a tuple:

:db.type/bigdec :db.type/bigint :db.type/boolean :db.type/double 
:db.type/instant :db.type/keyword :db.type/long :db.type/string 
:db.type/symbol :db.type/ref :db.type/uri :db.type/uuid

String values within a tuple are limited to 256 characters.

nil is a legal value for any slot in a tuple. This facilitates using tuples in range searches, where nil sorts lowest.

Datomic includes the query helpers tuple and untuple to support storing same values both in a tuple and in separate attributes.

Composite tuples are applicable:

• when a domain entity has a multi-attribute key
• to optimize a query that joins more than one high-population attribute on the same entity

For example, consider the domain of course registrations, modeled with the following entity types:

• courses represent a course, e.g. Algebra II
• semesters represent a period in time when a course is run, e.g. "Fall 2019"
• students can take courses in particular semesters

[{:db/ident :student/first
  :db/valueType :db.type/string
  :db/cardinality :db.cardinality/one}
 {:db/ident :student/last
  :db/valueType :db.type/string
  :db/cardinality :db.cardinality/one}
 {:db/ident :student/email
  :db/valueType :db.type/string
  :db/cardinality :db.cardinality/one
  :db/unique :db.unique/identity}
 {:db/ident :semester/year
  :db/valueType :db.type/long
  :db/cardinality :db.cardinality/one}
 {:db/ident :semester/season
  :db/valueType :db.type/keyword
  :db/cardinality :db.cardinality/one}
 {:db/ident :semester/year+season
  :db/valueType :db.type/tuple
  :db/tupleAttrs [:semester/year :semester/season]
  :db/cardinality :db.cardinality/one
  :db/unique :db.unique/identity}
 {:db/ident :course/id
  :db/valueType :db.type/string
  :db/unique :db.unique/identity
  :db/cardinality :db.cardinality/one}
 {:db/ident :course/name
  :db/valueType :db.type/string
  :db/cardinality :db.cardinality/one}]

A registration entity is a unique combination of a student, semester, and course. In Datomic schema:

{:db/ident :reg/course
 :db/valueType :db.type/ref
 :db/cardinality :db.cardinality/one}
{:db/ident :reg/semester
 :db/valueType :db.type/ref
 :db/cardinality :db.cardinality/one}
{:db/ident :reg/student
 :db/valueType :db.type/ref
 :db/cardinality :db.cardinality/one}

A given course/semester/student combination is unique in the database. To model this, you can create a composite tuple whose :db/tupleAttrs are

{:db/ident :reg/course+semester+student
 :db/valueType :db.type/tuple
 :db/tupleAttrs [:reg/course :reg/semester :reg/student]
 :db/cardinality :db.cardinality/one
 :db/unique :db.unique/identity}

With this composite installed, Datomic's unique identity will ensure that all assertions about a semester/course/student combination resolve to the same entity.

Composite attributes are entirely managed by Datomic–you never assert or retract them yourself. Whenever you assert or retract any attribute that is part of a composite, Datomic will automatically populate the composite value.

Given a database with the courses and semesters schema, add some seed data:

[{:semester/year 2018
  :semester/season :fall}
 {:course/id "BIO-101"}
 {:student/first "John"
  :student/last "Doe"
  :student/email "johndoe@university.edu"}]

Now if you register John for Bio 101 in the fall of 2018 by transacting:

[{:reg/course [:course/id "BIO-101"]
  :reg/semester [:semester/year+season [2018 :fall]]
  :reg/student [:student/email "johndoe@university.edu"]}] 

Datomic will also add the composite tuple datom:

#datom[20736789299855447 83 [6324390882967637 44112406506373204 64110323992363094] 13194139533320 true]]

Note that your entity IDs will differ from those in the example above

If the current value of an entity does not include all attributes of a composite, the missing attributes will be nil. For example, given a composite 4-tuple :reg/course+semester+student+grade that also includes a student’s grade, the assertions above would cause Datomic to populate:

#datom[20736789299855447 83 [6324390882967637 44112406506373204 64110323992363094 nil] 13194139533320 true]]

Note that nil sorts lower than all other values, so tuples with trailing nils can be useful for range queries.

If you retract all constituents of a composite, Datomic will retract the composite. For example, transacting:

[[:db/retract 20736789299855447 :reg/course [:course/id "BIO-101"]]
 [:db/retract 20736789299855447 :reg/semester [:semester/year+season [2018 :fall]]]
 [:db/retract 20736789299855447 :reg/student [:student/email "johndoe@university.edu"]]]

will cause Datomic to retract the composite:

#datom[20736789299855447 83 [6324390882967637 44112406506373204 64110323992363094] 13194139533321 false]]

Again, note that you will need to substitute the entity IDs from your initial transaction to replicate this example in your system.

Adding a composite tuple to a database that contains existing data using those attributes will not immediately generate values for the new tuple. The composite tuple will be created the next time any of the composite member attributes are transacted. This includes "no-op" transactions of the same attribute value. This design allows you to add composite tuples in a systematic and paced manner, so as not to overwhelm a running system.

An example helper function can be found in the day of datomic cloud examples. The function will cycle through all values for a given attribute, reasserting them, with the specified batch size and pause between batches. You can use this, or something like it, to systematically add composite tuples once you've created the necessary schema attribute(s).

Heterogeneous tuples have a :db/tupleTypes attribute, with a value specified as a vector of 2-8 scalar types.

For example, you could model a location in a 2D game with the following tuple attribute:

{:db/ident :player/location
 :db/valueType :db.type/tuple
 :db/tupleTypes [:db.type/long :db.type/long]
 :db/cardinality :db.cardinality/one}

You can then explicitly assert a player's location with a vector of the appropriate tuple types:

(def data [{:player/handle "Argent Adept"
            :player/location [100 0]}])

Homogeneous tuples provide variable length composites of a single attribute type. The :db.type of a homogeneous tuple is specified by the keyword attribute :db/tupleType.

Datomic itself includes a good example of homogeneous tuples in the definitions of the other tuple types. Both :db/tupleTypes and :db/tupleAttrs are declared as homogeneous tuples of :db/tupleType :db.type/keyword:

;; built in to Datomic
[{:db/ident :db/tupleAttrs
  :db/valueType :db.type/tuple
  :db/tupleType :db.type/keyword
  :db/cardinality :db.cardinality/one}
 {:db/ident :db/tupleTypes
  :db/valueType :db.type/tuple
  :db/tupleType :db.type/keyword
  :db/cardinality :db.cardinality/one}]

The :db.entity/attrs attribute is a multi-valued attribute of keywords, where each keyword names a required attribute.

For example, the following transaction data creates a spec that requires a :user/name and :user/email:

{:db/ident :user/validate
 :db.entity/attrs [:user/name :user/email]}

The :user/validate entity can then be used in later transaction to ensure that all required attribute are present. For example, the following transaction would fail

{:user/name "John Doe"
 :db/ensure :user/validate}

When a required attribute is missing, Datomic will throw an anomaly whose ex-data includes the failing entity and the name of the spec, e.g.:

{:cognitect.anomalies/category
 :cognitect.anomalies/incorrect, 
 :cognitect.anomalies/message
 "Entity 42 missing attributes [:user/email] of by :user/validate"}

The :db.entity/preds attribute is a multi-valued attribute of symbols, where each symbol names a predicate of database value and entity id. Inside a transaction, Datomic will call all predicates, and abort the transaction if any predicate returns a value that is not true.

For example, given the following predicate that has been deployed as an ion:

(ns datomic.samples.entity-preds
  (:require [datomic.api :as d]))

(defn scores-are-ordered?
  [db eid]
  (let [m (d/pull db [:score/low :score/high] eid)]
    (<= (:score/low m) (:score/high m))))

You could install the predicate on a guard entity along with an entity spec:

{:db/ident :score/guard
 :db.entity/attrs [:score/low :score/high] ;; entity spec
 :db.entity/preds 'datomic.samples.entity-preds/scores-are-ordered?} ;; entity predicate

Given an invalid entity that requests :score/guard:

{:score/low 100
 :score/high 20
 :db/ensure :score/guard}

Datomic will throw an anomaly whose ex-data names the failing predicate:

{:cognitect.anomalies/category 
 :cognitect.anomalies/incorrect,
 :cognitect.anomalies/message
 "Entity 42 failed pred datomic.samples.entity-preds/scores-are-ordered? of spec :score/guard"
 :db.error/pred-return false}

Datomic will report the value returned by the failing predicate under the :db.error/pred-return key. This can be used to report more information about what went wrong.

A full example can be found in Day of Datomic Cloud

cancel can be used directly from entity predicates to cancel the transaction.

Next: Changing Schema