How an entity's logical fields compose its single row key.
Bigtable's row key is one lexicographically-sorted string, and composing it
from several logical fields is the platform's standard practice. That is a
MAPPING concern rather than the composite-key CONTRACT concern M79 added:
the repository still addresses a row by an EntityKey, and this decides how
that key becomes bytes.
readonly
fields: readonly string[]
The logical fields the row key is composed from, in order. The order is load-bearing: it is both the byte order the key sorts in and the order the portable cursor carries key values in.
readonly
optional
separator: string
The separator joining the fields. Defaults to '#'. Ignored for a
single-field key, which is the field's own string form.
readonly
optional
prefix: string
A constant prefix prepended to every row key of this entity.