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hash

relationalai.semantics.std.common
hash(*args: Value) -> Expression

Compute a hash value for the given arguments.

Parameters

  • *args

    (Value, default: ()) - Values to hash together. At least one value is required.

Returns

  • Expression - An Expression representing the computed hash value. Returns Hash.

Raises

  • ValueError - If called with no arguments.

Examples

Compute hash of a single value:

select(common.hash(Person.email))

Compute hash of multiple values together:

select(common.hash(Order.customer_id, Order.order_date))

Notes

A missing argument means no hash, rather than a hash over the missing value. Where the hash is a row source — a where, a define, an aggregate — the row is eliminated; in a select the anchoring row keeps its place and the hash reads back as missing. Nothing is raised either way, so a model hashing a value that is absent for some rows loses those rows instead of computing an identity for them.

The value is not part of any compatibility guarantee and can change between releases. The common way to be caught by that is uuid_to_string(hash(...)) used as an identify_by value: after an upgrade the same source rows mint different entities, with nothing raised. Recompute stored hashes rather than comparing one against a hash computed by another version. It is not a cryptographic hash.

Do not rely on the hash separating types: some backends compute it over the arguments’ string forms, so hash(1) and hash("1") can compare equal.

Adjacent arguments stay distinct: hash("ab", "c") and hash("a", "bc") differ. An argument containing U+001F can collide with an argument boundary (hash("a\x1fb") can equal hash("a", "b")), so keep that byte out of the values you hash. An entity argument hashes as the entity’s identity, not its key columns. Where a backend cannot reconstruct that identity it raises at query time. The identity itself can still collide: an entity key that nests entities two or more levels deep, or a composite key whose adjacent parts concatenate alike (the identity encoding carries no separators).

Arguments of type Date, DateTime, Hash (which includes a nested hash), non-integer numbers, and entities are not rendered identically by every backend, so a hash taken over them is not portable across backends. For Date, DateTime, Hash, and non-integer numbers this is the same restriction that already applies to entity construction.