Entity adapters: Managing nested relationships and cascading deletes

Answer

Model the parent-child link with a foreign key on the child (for example, child.parentId), then implement cascading deletes in a custom reducer that removes matching children and the parent in a single immutable update. There is no built-in ngrx/entity mechanism for nested relationships, and there is no officially endorsed linking pattern beyond explicit foreign-key IDs.

Confirmed facts

• ngrx/entity adapters manage one flat entity collection per adapter; they do not track relationships between collections.
• Referential integrity must be enforced manually by filtering child records before or while removing the parent.
• A non-strict sortComparer that returns 0 for equal business values can allow unstable ordering because tied items may swap positions across sort operations.

Likely explanation

The adapter generates collection-level CRUD reducers only, so cascading behavior requires a helper selector and a custom delete case. Stability is restored by making the comparer a strict weak ordering: when the primary field ties, fall back to a unique immutable field such as entity.id.

Steps for this case

1. Add parentId to the child entity interface.
2. Create a selector such as selectChildrenByParentId that filters the child collection by parentId.
3. In the parent delete reducer, compute the children to remove, then return a state with both the parent and those children filtered out.
4. Define sortComparer to compare the business field first, then compare id when values are equal.

const selectChildrenByParentId = createSelector(
  selectAllChildren,
  (children, parentId) => children.filter((c) => c.parentId === parentId)
);

// In the parent reducer delete case
const childrenToRemove = selectChildrenByParentId(state, action.payload.parentId);
const nextChildIds = state.childIds.filter(
  (id) => !childrenToRemove.some((c) => c.id === id)
);
return {
  ...state,
  parentIds: state.parentIds.filter((id) => id !== action.payload.parentId),
  childIds: nextChildIds
};

const sortComparer = (a, b) => {
  if (a.businessField < b.businessField) return -1;
  if (a.businessField > b.businessField) return 1;
  if (a.id < b.id) return -1;
  if (a.id > b.id) return 1;
  return 0;
};

Missing diagnostic detail

Confirm whether children store a direct parentId or whether the relationship lives in a separate lookup table or is many-to-many. If it is not a direct foreign key, the cascade logic must be redesigned.

Verification

• After deleting a parent, selectAllChildren filtered by that parentId returns an empty array.
• Unit-test the reducer by dispatching the delete action and asserting the result equals a manually constructed state with the parent and children removed.
• Insert entities with equal businessField values but different IDs, then confirm their relative order stays fixed across repeated reducer updates.

Assumptions: entities have a unique id field, updates are synchronous within a single reducer, and the store uses ngrx/entity selectors generated from the adapter.