event-sourcing-aggregate-validation

name	event-sourcing-aggregate-validation
description	Use when defining event-sourced aggregate validation at command time, hook time, save time, or for computed snapshot/query mirror properties.
category	architecture
roles	["architecture","coding","domain-driven-design"]
tags	["event-sourcing","aggregate","validation","fluentvalidation","snapshots"]

Event Sourcing Aggregate Validation Skill

Use this skill when defining validation strategy for event-sourced aggregates in Purview.EventSourcing.

Validation layers to apply

1. Command/event-raising validation

   • Enforce transition and intent rules in generated hooks (OnComputing...Event, OnRaising...Event, OnShouldApply...Event, On<Property>Changing).
   • Reject invalid operations before events are recorded.

2. Aggregate model validation

   • Use DataAnnotations on aggregate properties for structural constraints.
   • Examples: [Range], [Required], length constraints.

3. Save-time validation

   • Save pipeline validates aggregates through IAggregateValidator<TAggregate>.
   • If no custom validator is provided, DefaultAggregateValidator<TAggregate> runs DataAnnotations validation.
   • If a FluentValidation IValidator<TAggregate> is provided, it is adapted via FluentValidationAggregateValidator<TAggregate>.

4. Computed mirror validation

   • For snapshot/query-facing mirror properties, derive values in [Computed] hooks instead of trusting caller input.
   • Keep canonical domain properties as the source for recomputing mirrored query shapes.

Rules

• Keep invariant checks close to mutation points (command/hook layer).
• Keep structural and shape constraints declarative (DataAnnotations/FluentValidation).
• Do not silently swallow validation failures.
• Treat SaveResult<TAggregate>.IsValid and ValidationResult as first-class outcomes.
• Use SaveResult.EnsureValid() when invalid saves must throw immediately.
• Preserve replay tolerance by avoiding command-time assumptions in hydration-only paths.
• For computed snapshot/query mirrors, validate derivation rules, not user-supplied values.
• If a save-time validator is added, prefer IAggregateValidator<TAggregate> over ad-hoc validation inside stores.

Aggregate validation checklist

• Every command has explicit precondition checks.
• Property transitions are guarded (On<Property>Changing / On<Property>Changed where needed).
• Aggregate-level attributes capture simple declarative constraints.
• Custom FluentValidation rules are used for cross-field/business policies not suited to attributes.
• Save handlers inspect or enforce SaveResult.ValidationResult.
• Query-facing mirror properties are recomputed consistently from canonical state.

Output template to use

1. Validation map by layer (command, aggregate model, save pipeline, computed mirrors).
2. Hook methods and invariants per command (OnComputing...Event, OnRaising...Event, OnShouldApply...Event, On<Property>Changing/Changed).
3. Attribute-based constraints and rationale.
4. FluentValidation rules (if any) and when they execute.
5. SaveResult handling policy (IsValid, EnsureValid, error propagation).
6. Mirror-property derivation guarantees when query-facing fields exist.