$db Query Guide

$db is the function-tier runtime handle for reading and writing the cloud-persisted Mongo-backed storage components. Predicates use a fluent operator DSL that's compiled to a server-side query — not raw JavaScript evaluation. This is the single most important thing to understand before writing $db code, because native operators compile without error and silently misbehave at runtime.

This skill is read-only consultation: it owns the $db mechanics (predicate DSL, operators, schema-change audit) and the flow-datasource-over-$db sub-variant. It does not author functions, datasources, or storage components on its own — those round-trips are owned by the parent creator skill that invoked this one (typically function-creator).

See also: function-creator — authoring the backend flow that hosts the $db call. db-query owns the predicate DSL and storage-access mechanics; function-creator owns the function lifecycle (dxs function generate / validate / upsert).

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