Serialize derived attributes during xml and key_value serialization

[HassanAkbar]

fixes #639

[Copilot]

Pull request overview

Fixes #639 by ensuring derived (method:) attributes are not incorrectly treated as “defaulted” after XML deserialization, so they continue to serialize in downstream formats.

Changes:

• Add derived-attribute detection to bypass “skip default” logic in RenderPolicy.
• Ensure MappingRule#render? renders derived attributes even when using_default? is true.
• Add an RSpec covering derived attribute serialization after XML deserialization (YAML + JSON/key-value).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
lib/lutaml/model/render_policy.rb	Avoids skipping derived attributes due to default-render policy.
lib/lutaml/model/mapping/mapping_rule.rb	Forces derived attributes to render even when instance reports “using default”.
spec/lutaml/model/derived_attribute_serialization_spec.rb	Adds regression coverage for YAML + JSON after XML deserialization.

Comments suppressed due to low confidence (1)

lib/lutaml/model/render_policy.rb:100

• should_skip_default? now calls derived_attribute? unconditionally, which does an attribute lookup on every non-nil/non-empty value even when using_default? is false (the common case for instances created via .new). To avoid a potential serialization hot-path regression, consider only checking derived_attribute? inside the existing using_default? && !render_default branch (i.e., only when the rule would otherwise be skipped due to defaults).

      def should_skip_default?(value, rule, context_obj, attr_name)
        return false if derived_attribute?(context_obj, attr_name)

        # Skip if context object is using default and render_default is false
        # But for collections, check if they were mutated (non-empty)
        if context_obj.respond_to?(:using_default?) &&
            context_obj.using_default?(attr_name) &&
            !extract_option(rule, :render_default)
          # For collections: if mutated to non-empty, serialize them

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[ronaldtse]

For custom classes defined in specs, we should wrap this in a module just for this spec to prevent pollution. We should add this rule to CLAUDE.md or AGENT.md. Or, use anonymous classes.

[HassanAkbar]

@ronaldtse wrapped the class inside a module to prevent pollution. Let me know if you want me to change anything else. Thanks.

[ronaldtse]

@HassanAkbar LGTM feel free to merge when ready.

[HassanAkbar]

@ronaldtse Sure, I’ll merge it once the CI is green.