Schema Data Reference

This document defines the grammar for schema data.

Schema Grammar

Syntax Used in Grammar

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

Schema Map Grammar

attr-def    = {':db/ident' keyword
               ':db/cardinality' cardinality
               ':db/valueType' type
               (':db/doc' string)?
               (':db/unique' unique)?
               (':db/isComponent' boolean)?
               (':db/id' tx-entid)?
               (':db/noHistory' boolean)?
               (':db.attr/preds' attr-pred+)?
               (':db.entity/attrs' ident+)?
               (':db.entity/preds' ent-pred+)?}
tx-entid = (identifier | tempid)
tempid = string
identifier = (eid | lookup-ref | ident)
eid = nat-int
lookup-ref = [identifier value]
ident = keyword
attr-pred = qualified symbol naming a /predicate/ of a value
ent-pred = qualified symbol naming a /predicate/ of db-after and an eid
cardinality = (':db.cardinality/one' | ':db.cardinality/many')
type = (':db.type/bigdec' | ':db.type/bigint' | ':db.type/boolean' | ':db.type/bytes' | ':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')

Grammar Notes

The grammar above shows only the attributes that are built-in to Datomic schema. Just like all other entities in Datomic, schema entities are open and can have any attributes you define added to them.

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.

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

Defining Schema

Attributes are defined using the same data model used for application data. That is, attributes are themselves defined by entities with associated attributes and are added to the database using d/transact.

Name Purpose Required?
:db/ident specifies a unique programmatic name for an entity (normally a schema entity) Required for schema entities
:db/cardinality specifies whether an attribute associates a single value or a set of values Required
:db/valueType specifies the type of value that can be associated with an attribute Required
:db/unique specifies a uniqueness constraint for the values of an attribute Optional
:db/isComponent specifies whether an attribute is a ref to a component entity Optional
:db/noHistory specifies whether historical values should be forgotten for an attribute Optional
:db/doc specifies a documentation string for an attribute Optional
:db.attr/preds specifies one or more predicates that constrain an attribute's value by more than just its value type Optional

The example map form below shows an attribute that represents a person's name:

{:db/ident       :person/name
 :db/valueType   :db.type/string
 :db/cardinality :db.cardinality/one
 :db/doc         "A person's name"}


{':db/cardinality' cardinality}
cardinality = (':db.cardinality/one' | ':db.cardinality/many')

The required :db/cardinality attribute specifies whether an attribute associates a single value or a set of values with an entity. It has no default value.

The values allowed for :db/cardinality are:

  • :db.cardinality/one – the attribute is single-valued, it associates a single value with an entity.
  • :db.cardinality/many – the attribute is multi-valued, it associates a set of values with an entity.


':db/doc' = string

The optional :db/doc specifies a documentation string, and can be any string value.


{':db/id' tx-entid}
tx-entid       = (identifier | tempid)
identifier     = (eid | lookup-ref | ident)
eid            = nat-int
lookup-ref     = [identifier value]
ident          = keyword

:db/id is not an attribute; rather, it is syntactic sugar for specifying the entity identifier in a map form. For example, the following two forms are equivalent:

{:db/id "alan"
 :person/name "Alan Turing"}

[:db/add "alan" :person/name "Alan Turing"]


{':db/ident' keyword}

The :db/ident attribute specifies a unique programmatic name for an entity. Idents are required for schema entities and are optional for all other entities.

Idents should be used for two purposes: to name schema entities and to represent enumerated values. To support these usages, idents are designed to be extremely fast and always available. All idents associated with a database are stored in memory in every Datomic compute node.

When an entity has an ident, you can use that ident in place of the eid, e.g.

;; assuming that :person/loves is entity id 1007, these are equivalent
[:db/add "alan" :person/loves "pizza"]
[:db/add "alan" 1007 "pizza"]

These characteristics also imply situations where idents should not be used:

  • Idents should not be used as unique names or ids on ordinary domain entities. Such entity names should be implemented with a domain-specific attribute that is a unique identity.
  • Idents should not be used as names for test data. Your real data will not have such names, and you don't want test data to behave differently than the real data it simulates.

Idents can be used instead of entity ids in the following API calls:

  • As the sole argument to d/entity
  • In the E, A, and V positions of assertions and retractions passed to d/transact and d/with
  • In the E, A, and V positions of a where clause in a query

Allowable Values

The allowable value of :db/ident is a Clojure keyword. It is idiomatic to namespace-qualify all idents you define. 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.

If using underscores in :db/ident values, do not use as the first character in the name portion of the keyword as this will prevent you from using reverse lookups.


{':db/isComponent' boolean}

A component entity is one that exists only as part of a larger parent entity.

The optional :db/isComponent attribute specifies that an attribute whose :db/valueType is :db.type/ref refers to a sub-component of the entity to which the attribute is applied. When you retract an entity with :db.fn/retractEntity, all sub-components are also retracted.

Omitting :db/isComponent for an entity is semantically equivalent to setting it to false.


{':db/noHistory' boolean}

Description and Use Cases

By default, Datomic maintains all historical values of an attribute. To disable this, set :db/noHistory to true. The purpose of :db/noHistory is to conserve storage, not to make semantic guarantees about removing information.

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


{':db/unique' unique}
unique         = (':db.unique/identity' | ':db.unique/value')

The :db/unique attribute specifies a uniqueness constraint for the values of an attribute. Datomic will reject a transaction if the resulting database would contain multiple entities with the same value for a unique attribute (of either type). You can submit transaction data with a unique attribute without specifying an existing entity id. When you do, one of two things could happen if that unique AV already exists in the database: unify or reject.

To add a uniqueness constraint to an attribute:

  • The attribute must have a :db/cardinality of :db.cardinality/one
  • If there are values present for that attribute, they must be unique in the set of current database assertions

Adding a unique constraint does not change history, therefore historical databases may contain non-unique values. Code that expects to find a unique value may find multiple values when querying against history.


Unique identity is specified through an attribute with :db/unique set to :db.unique/identity. Unique identity is appropriate whenever you want to assert a database-wide unique identifier for an entity. Common use cases include email addresses, account names, product codes/skus, and UUIDs. If transaction data includes a tempid + unique identity, and an entity with that identity already exists in the database, Datomic will unify the new transaction data with the existing entity id. This enables "upsert", e.g.

{:db/ident       :person/email
 :db/unique      :db.unique/identity
 :db/valueType   :db.type/string
 :db/cardinality :db.cardinality/one}

;; existing in db
[42 :person/email "johndoe@example.com"]

;; transaction data in map form
{:db/id        "upsert-age"
 :person/email "johndoe@example.com"
 :person/age   50}

;; example of what map form might become after resolving tempid via unique identity
[:db/add 42 :person/age 50]

An entity can have multiple different unique attributes, however, this creates the possibility of encountering a conflict anomaly. If a transaction tries to upsert a tempid into two different existing entities. For 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 entity has both an email of johndoe@example.com and an account number of 1007.


Unique value is specified through an attribute with :db/unique set to :db.unique/value. If transaction data includes a tempid + unique value, and an entity with that value already exists, Datomic will reject the transaction.


{':db/valueType' type}
type           = (':db.type/bigdec'  | ':db.type/bigint'  | ':db.type/boolean' |
                  ':db.type/bytes'   | ':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')

The :db/valueType attribute specifies the type of value that can be associated with an attribute. The type is one of the keywords in the table below.

:db/valueType cannot be updated after an attribute is created.

Value type Description Java equivalent Example
:db.type/bigdec Arbitrary precision decimal java.math.BigDecimal 1.0M
:db.type/bigint Arbitrary precision integer java.math.BigInteger 7N
:db.type/boolean Boolean boolean True
:db.type/bytes Value type for small binary data byte[] (byte-array (map byte [1 2 3]))
:db.type/double 64-bit IEEE 754 floating point number double 1.0
:db.type/float 32-bit IEEE 754 floating point number float 1.0
:db.type/instant Instant in time java.util.Date #inst "2017-09-16T11:43:32.450-00:00"
:db.type/keyword Namespace + name N/A :yellow
:db.type/long 64 bit two's complement integer long 42
:db.type/ref Reference to another entity N/A 42
:db.type/string Unicode string java.lang.String "Foo"
:db.type/symbol Symbol N/A Foo
:db.type/tuple tuples of scalar values N/A [42 12 "foo"]
:db.type/uuid 128-bit universally unique identifier java.util.UUID #uuid "f40e770e-9ad5-11e7-abc4-cec278b6b50a"
:db.type/uri Uniform Resource Identifier (URI) java.net.URI https://www.datomic.com/details.html

Notes on Value Types

  • 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.

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 for working with tuples in queries.

Composite Tuples

Composite tuples are applicable in the following situations:

  • 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 Composites to Existing Entities

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 populated 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 pausing 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

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/handle
  :db/valueType   :db.type/string
  :db/cardinality :db.cardinality/one
  :db/unique      :db.unique/identity}
 {: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:

[{:player/handle   "Argent Adept"
  :player/location [100 0]}]

Homogeneous Tuples

Homogeneous tuples provide variable-length composites of a single attribute type. The 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}]

Attribute Predicates

You may want to constrain an attribute value by more than just its value type. For example, an email address is not just a string, but a string with a particular format. In Datomic, you can assert attribute predicates about an attribute. Attribute predicates are asserted via the :db.attr/preds attribute, and are fully-qualified symbols that name a predicate of a value. Predicates return true (and only true) to indicate success. All other values indicate failure and are reported back as transaction errors.

Inside transactions, Datomic will call all attribute predicates for all attribute values, and abort a transaction if any predicate fails.

For example, the following function validates that a user-name has a particular length:

(ns datomic.samples.attr-preds)

(defn user-name?
  (<= 3 (count s) 15))

To install the user-name? predicate, add a db.attr/preds value to an attribute, e.g.

{:db/ident :user/name,
 :db/valueType :db.type/string,
 :db/cardinality :db.cardinality/one,
 :db.attr/preds 'datomic.samples.attr-preds/user-name?}

A transaction that includes an invalid user-name will result in an incorrect anomaly that includes:

  • The entity id
  • The attribute name
  • The attribute value
  • The name of the failed predicate
  • The predicate return in :db.error/pred-return

For example, the string "This-name-is-too-long" is not a valid user-name? and will cause an anomaly like:

 "Entity 42 attribute :user/name value This-name-is-too-long
failed pred datomic.samples.attr-preds/user-name?"
 :db.error/pred-return false}

Attribute predicates must be on the classpath of a process that is performing a transaction.

Attribute predicates can be asserted or retracted at any time, and will be enforced starting on the transaction after they are asserted. Asserting or retracting an attribute predicate does not affect attribute values that already exist in the database.

Attribute predicates can cancel the transaction directly.

Entity Specs

You may want to ensure properties of an entity being asserted, for example:

  • Required keys
  • The creation of related tuples
  • Satisfaction of properties that cut across attributes and the DB

An entity spec is a Datomic entity having one or more of:

You can then ensure an entity spec by asserting the :db/ensure attribute for an entity. For example, the following transaction data ensures entity "new-account-1" with entity spec :new-account:

{:db/id "new-account-1"
 :db/ensure :new-account
 ;; other data that must be a valid :new-account

:db/ensure is a virtual attribute. It is not added in the database; instead, it triggers checks based on the named entity.

Entity predicates must be on the classpath of a process that is performing a transaction.

Entity specs can be asserted or retracted at any time and will be enforced starting on the transaction after they are asserted. Asserting or retracting an entity spec has no effect on entities that already exist in the database since :db/ensure only triggers checks without creating extra transaction data.

Entity specs and :db/ensure are not analogous to traditional SQL constraints:

  • Specs are more flexible, enforcing arbitrary shapes on particular entities without imposing an overall structure on the database
  • Specs can do more, allowing arbitrary functions of an entity and the database value as of the start of the transaction (db-before)
  • Spec enforcement for an entity must be requested explicitly at transaction time via :db/ensure; enforcement is never automatic or retroactive

Required Attributes

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 attributes 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.:

 "Entity 42 missing attributes [:user/email] of spec :user/validate"}

Entity Predicates

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:

(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 required attributes:

{:db/ident        :score/guard
 :db.entity/attrs [:score/low :score/high] ;; required attributes
 :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:

 "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

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

Limitations of NaN

The comparison semantics of NaN (Java's Float and Double constant holding Not-A-Number) make it unavailable for upsert. Upsert in Datomic requires the ability to compare the desired new value to any existing value, which cannot be done when the existing value is NaN. As such, to assert a new value for an attribute whose current value is NaN, a retraction must first be transacted for that NaN value, then a new value can be asserted.


This section contains information about features that have been deprecated. These features will not be removed from the product, but you are discouraged from using them for new development.

Limitations of Bytes

The :db.type/bytes value type has been deprecated because it maps directly to Java byte arrays, which do not have value semantics (semantically equal byte arrays do not compare or hash as equal). As a result, a bytes attribute can function only as a container of data that is semantically opaque, i.e. given an entity id you can look up a bytes attribute value

Bytes attributes cannot be used in situations that require value semantics:

  • Cannot be unique and therefore cannot be used to lookup an entity
  • Cannot be used in Datomic Cloud
  • Cannot be used with analytics