>>> [!note] Migrated issue <!-- Drupal.org comment --> <!-- Migrated from issue #3588494. --> Reported by: [jurgenhaas](https://www.drupal.org/user/168924) Related to !28 >>> <h3>Problem/Motivation</h3> <p>The successor model in <code>modeler_api</code> permits multiple successors between the same source and target component. Each successor carries an optional <code>conditionId</code> (see <code>Drupal\modeler_api\ComponentSuccessor</code>).</p> <p>For some model owners &mdash; for example ECA, see the related ECA issue &mdash; multiple parallel successors only make sense when every one of them carries a condition. Without conditions on all of them, the runtime semantics are degenerate (the same target would be invoked multiple times unconditionally) and the model is structurally meaningless.</p> <p>Today the constraint system in <code>Drupal\modeler_api\Api::validateModelConstraints()</code> can enforce per-type counts and per-component successor counts, but it cannot express the rule "if two-or-more successors share the same source and target, every one of them must carry a condition".</p> <p>This issue is a follow-up to <a href="https://www.drupal.org/project/modeler/issues/3586864">modeler #3586864</a>, which made parallel edges visually distinguishable. The rule defined here ensures that the parallel edges saved into a model are also semantically meaningful &mdash; but only for model owners that opt in.</p> <h3>Proposed resolution</h3> <p>Extend <code>CardinalityConstraint</code> with an optional, opt-in flag on the per-type <code>successors</code> entry:</p> <pre><pre>[<br>&nbsp; Api::COMPONENT_TYPE_ELEMENT =&gt; [<br>&nbsp;&nbsp;&nbsp; 'successors' =&gt; [<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 'min' =&gt; 0,<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 'max' =&gt; 10,<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 'requireConditionWhenParallel' =&gt; TRUE,&nbsp; // &larr; new<br>&nbsp;&nbsp;&nbsp; ],<br>&nbsp; ],<br>]</pre></pre><p>When a model owner sets <code>requireConditionWhenParallel</code> to <code>TRUE</code> for a given component type, <code>Drupal\modeler_api\Api::validateModelConstraints()</code> applies the following check for every component of that type:</p> <blockquote><p>Group the component's successors by target id. For every group of size &gt; 1, every successor in the group must have a non-empty <code>conditionId</code>; otherwise validation fails with a translated error message that names the source and target.</p> </blockquote> <p>Other model owners are unaffected &mdash; the default flag value is <code>FALSE</code>, so no existing behavior changes.</p> <p>The flag is forwarded to the frontend as part of the existing <code>model_constraints</code> settings payload (<code>Api::prepareModelConstraints()</code>), so the modeler UI can mirror the check in <code>validateBeforeSave()</code> for fast pre-save feedback.</p> <h3>Implementation outline</h3> <ol> <li>Update the docblock and example on <code>ModelOwnerInterface::modelConstraints()</code> to document the new flag.</li> <li>Add the validation branch in <code>Api::validateModelConstraints()</code>: when <code>requireConditionWhenParallel</code> is true for a source component type, group that component's successors by target id and verify every group of size &gt; 1 has a condition on every member.</li> <li>Pass the flag through <code>Api::prepareModelConstraints()</code> unchanged so the frontend receives it.</li> <li>Unit tests covering: opt-in true (rule enforced), opt-in false / unset (rule not enforced), single successor unaffected, two parallel successors with all conditions allowed, two parallel successors with any one missing a condition rejected.</li> <li>Update <code>modeler_api</code> documentation to describe the flag.</li> </ol> <h3>Remaining tasks</h3> <ul> <li>Implement the validation branch in <code>Api</code>.</li> <li>Update the interface docblock and example.</li> <li>Unit tests.</li> <li>Documentation update.</li> </ul> <h3>User interface changes</h3> <p>None in <code>modeler_api</code> directly. The modeler UI mirror is tracked in the related <code>modeler</code> issue.</p> <h3>API changes</h3> <ul> <li>New optional flag <code>requireConditionWhenParallel</code> on the <code>successors</code> sub-shape of <code>CardinalityConstraint</code>. Default <code>FALSE</code>; opt-in per source component type.</li> <li>No change to <code>ModelOwnerInterface</code> method signatures.</li> <li>No change to <code>ComponentSuccessor</code>.</li> </ul> <h3>Data model changes</h3> <p>None. <code>ComponentSuccessor.conditionId</code> already carries the data the validator consults.</p> <h3>Related issues</h3> <ul> <li>Modeler UI mirror: <em>(link to modeler issue once filed)</em></li> <li>ECA opt-in: <em>(link to ECA issue once filed)</em></li> <li>Originating context: <a href="https://www.drupal.org/project/modeler/issues/3586864">modeler #3586864</a></li> </ul>