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Cratis Cratis Cratis

CHR0030: [ChildrenFrom] child collection property auto-maps to nothing

Rule Description

Section titled “Rule Description”

A collection property on a [ChildrenFrom] child is filled by AutoMap, which matches the event’s property of the same name. When no source event carries a matching property and nothing maps it explicitly, the property silently projects as an empty collection. Rename the property to match the event, or bridge the names with [SetFrom<TEvent>(nameof(...))].

Severity

Section titled “Severity”

Warning

Example

Section titled “Example”
using Cratis.Chronicle.Events;
using Cratis.Chronicle.Keys;
using Cratis.Chronicle.Projections.ModelBound;


public record Chr0033Note(string Text);


[EventType]
public record Chr0033LineAdded(string LineNumber, string Description, IReadOnlyList<Chr0033Note> Annotations);


public record Chr0033Line(
    [Key] string LineNumber,
    string Description,
    IReadOnlyList<Chr0033Note> Notes);


public record Chr0033Order(
    [Key] Guid Id,


    // Warning CHR0030: the child property 'Notes' matches no property on Chr0033LineAdded (the event
    // carries 'Annotations'), so AutoMap fills it from nothing and it always projects as empty.
    [ChildrenFrom<Chr0033LineAdded>(key: nameof(Chr0033LineAdded.LineNumber))]
    IEnumerable<Chr0033Line> Lines);

Why This Rule Exists

Section titled “Why This Rule Exists”

A [ChildrenFrom] child is projected from its own event, and each of its properties is filled by AutoMap — Chronicle copies the event property of the same name onto it. When the child declares a collection property whose name matches no property on that event, and no [SetFrom] (or other mapping attribute) bridges the gap, there is nothing for AutoMap to copy. The child still projects, but that collection is always empty — a silent data loss rather than a compile or runtime error.

There are two fixes, depending on which name is right:

  • Rename the property to match the event — call the child property what the event calls it, and AutoMap wires it automatically.
  • Bridge the names explicitly — keep the property name and point it at the event property with [SetFrom<TEvent>(nameof(TEvent.Property))].

This is the child-collection mirror of the scalar mapping-source rules: the same empty-projection trap the runtime projection engine also reports as a warning, surfaced here at build time so it never reaches production data.

Section titled “Related Rules”
  • CHR0024 — the scalar version: a read model property with no mapping source that AutoMap cannot populate.
  • CHR0025 — the inverse collision: an explicitly sourced property that AutoMap overwrites from another event.
  • Model-bound projections — the full reference.