Unique Constraints (`f:enforceUnique`)

Fluree supports transaction-time enforcement of property value uniqueness via f:enforceUnique. This is complementary to SHACL — it runs independently.

How it works

Unique constraint enforcement has two parts:

1. Annotation: Mark properties as unique by asserting f:enforceUnique true on their IRIs in any graph
2. Activation: Enable enforcement in the config graph via f:transactDefaults

This separation follows the same pattern as SHACL (shapes + config activation) and reasoning (schema + config activation). Annotations alone do nothing — enforcement must be explicitly enabled.

Step 1: Define unique properties

Assert f:enforceUnique true on any property IRI that should enforce uniqueness. These annotations can live in the default graph or any named graph:

@prefix f: <https://ns.flur.ee/db#> .
@prefix ex: <http://example.org/ns/> .

# In the default graph
ex:email f:enforceUnique true .
ex:ssn   f:enforceUnique true .

Step 2: Enable enforcement

Enable unique constraint checking in the config graph:

@prefix f: <https://ns.flur.ee/db#> .

GRAPH <urn:fluree:mydb:main#config> {
  <urn:fluree:mydb:main:config:ledger> a f:LedgerConfig ;
    f:transactDefaults [
      f:uniqueEnabled true
    ] .
}

When f:constraintsSource is omitted, the default graph is used as the annotation source.

Explicit constraint source

To read annotations from a specific graph:

@prefix f: <https://ns.flur.ee/db#> .

GRAPH <urn:fluree:mydb:main#config> {
  <urn:fluree:mydb:main:config:ledger> a f:LedgerConfig ;
    f:transactDefaults [
      f:uniqueEnabled true ;
      f:constraintsSource [
        a f:GraphRef ;
        f:graphSource [ f:graphSelector f:defaultGraph ]
      ]
    ] .
}

Multiple constraint sources

Multiple sources can be specified — all are checked:

f:transactDefaults [
  f:uniqueEnabled true ;
  f:constraintsSource [
    a f:GraphRef ;
    f:graphSource [ f:graphSelector f:defaultGraph ]
  ] , [
    a f:GraphRef ;
    f:graphSource [ f:graphSelector <http://example.org/schema> ]
  ]
] .

What gets enforced

Once enabled, any transaction that would result in two or more distinct subjects holding the same value for a unique property within the same graph is rejected.

Scoping: per-graph

Uniqueness is enforced per graph. The same value on the same property is allowed across different named graphs:

# Graph A: ex:alice ex:email "alice@example.com" — OK
# Graph B: ex:bob   ex:email "alice@example.com" — OK (different graph)
# Graph A: ex:carol ex:email "alice@example.com" — REJECTED (same graph as alice)

Value identity

Uniqueness is determined by the storage-layer value representation, not by RDF strict equality. The uniqueness key is:

(graph, predicate, value)

where "value" is the internal storage representation (type discriminant + payload).

The enforcement query matches on (predicate, object) in the POST index without constraining by datatype or language tag. This means:

• Two values with different datatype IRIs but the same internal representation are treated as the same value. For example, "hello"^^xsd:string and "hello"^^ex:customType both store as the same string value internally, so they conflict.
• Two values with different language tags but the same string content conflict, because the language tag is metadata, not part of the value key.
• Two values with different internal representations are naturally distinct. For example, "42" (stored as a string) and 42 (stored as an integer) do not conflict because they are different value types at the storage layer.

This design matches how humans think about value identity and prevents circumventing uniqueness by attaching a different datatype annotation or language tag.

Intra-transaction enforcement

Uniqueness is checked after staging, so conflicts within a single transaction are caught:

{
  "@context": { "ex": "http://example.org/ns/" },
  "@graph": [
    { "@id": "ex:alice", "ex:email": "same@example.com" },
    { "@id": "ex:bob",   "ex:email": "same@example.com" }
  ]
}

This transaction is rejected because two subjects assert the same value for a unique property.

Upsert safety

Upserts that change a value are handled correctly. When an upsert retracts the old value and asserts a new one in the same transaction, the old value is no longer active — no false positive.

Idempotent re-insert

Re-asserting the same (subject, property, value) triple that already exists is allowed. One subject still holds the value — no violation.

Error message

When a uniqueness violation is detected, the transaction fails with an error like:

Unique constraint violation: property <http://example.org/ns/email>
  value "alice@example.com" already exists for subject
  <http://example.org/ns/alice> in graph default
  (conflicting subject: <http://example.org/ns/bob>)

Lagging config

Config is read from the pre-transaction state. This means:

• Enabling f:uniqueEnabled and inserting duplicate values in the same transaction will not reject the duplicates
• The next transaction will enforce the constraint

This is intentional and consistent with all other config graph features.

Per-graph overrides

Transact defaults use additive merge semantics:

• f:uniqueEnabled uses monotonic OR — once enabled at the ledger level, per-graph configs cannot disable it
• f:constraintsSource is additive — per-graph sources are added to (not replace) ledger-wide sources

Note: additive merge is still subject to override control. If the ledger-wide f:overrideControl for f:transactDefaults is f:OverrideNone, per-graph additions are blocked entirely.

This means a per-graph override can add additional constraint sources but cannot remove ledger-wide ones:

GRAPH <urn:fluree:mydb:main#config> {
  <urn:fluree:mydb:main:config:ledger> a f:LedgerConfig ;
    f:transactDefaults [
      f:uniqueEnabled true ;
      f:constraintsSource [
        a f:GraphRef ;
        f:graphSource [ f:graphSelector f:defaultGraph ]
      ]
    ] ;
    f:graphOverrides (
      [ a f:GraphConfig ;
        f:targetGraph <http://example.org/graphX> ;
        f:transactDefaults [
          f:constraintsSource [
            a f:GraphRef ;
            f:graphSource [ f:graphSelector <http://example.org/schema> ]
          ]
        ]
      ]
    ) .
}

In this example, graphX checks unique annotations from both the default graph (ledger-wide) and http://example.org/schema (per-graph addition).

Zero cost when not configured

When f:uniqueEnabled is not set or is false, uniqueness checking is completely skipped — no property scan, no index queries, no overhead. The enforcement code fast-paths out immediately.

Complete example

@prefix f:  <https://ns.flur.ee/db#> .
@prefix ex: <http://example.org/ns/> .

# 1. Define unique annotations in the default graph
ex:email f:enforceUnique true .

# 2. Enable enforcement in the config graph
GRAPH <urn:fluree:mydb:main#config> {
  <urn:fluree:mydb:main:config:ledger> a f:LedgerConfig ;
    f:transactDefaults [
      f:uniqueEnabled true
    ] .
}

After this transaction, the next transaction that attempts to give two subjects the same ex:email value (within the same graph) will be rejected.