javascript 47 lines · 8 steps

Diffing sets to sync group membership

Compute the minimal add/remove delta between two id lists, then apply only the changes inside a transaction.

Explained by highlit
Intermediate set-operations diffing transactions idempotency
1export function diffIds(current, desired) {
2 const currentSet = new Set(current);
3 const desiredSet = new Set(desired);
4 
5 const toAdd = [];
6 const toRemove = [];
7 
8 for (const id of desiredSet) {
9 if (!currentSet.has(id)) toAdd.push(id);
10 }
11 
12 for (const id of currentSet) {
13 if (!desiredSet.has(id)) toRemove.push(id);
14 }
15 
16 const unchanged = current.filter((id) => desiredSet.has(id));
17 
18 return {
19 toAdd,
20 toRemove,
21 unchanged,
22 hasChanges: toAdd.length > 0 || toRemove.length > 0,
23 };
24}
25 
26export async function syncMembership(groupId, desiredUserIds) {
27 const existing = await db.groupMembers.findAll({ where: { groupId } });
28 const { toAdd, toRemove, hasChanges } = diffIds(
29 existing.map((m) => m.userId),
30 desiredUserIds,
31 );
32 
33 if (!hasChanges) return { changed: false };
34 
35 await db.transaction(async (tx) => {
36 if (toRemove.length) {
37 await tx.groupMembers.destroy({ where: { groupId, userId: toRemove } });
38 }
39 if (toAdd.length) {
40 await tx.groupMembers.bulkCreate(
41 toAdd.map((userId) => ({ groupId, userId })),
42 );
43 }
44 });
45 
46 return { changed: true, added: toAdd, removed: toRemove };
47}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Wrapping id lists in Sets turns membership checks into O(1) lookups, keeping the diff linear.
  2. 2Computing a delta first lets you skip work entirely when nothing has changed.
  3. 3Applying removes and adds inside one transaction keeps the final state atomic and consistent.

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