How to Model Relationships in Firestore

1import { collection, addDoc, getDocs, query, orderBy, serverTimestamp } from 'firebase/firestore'
2
3// Add a comment to a specific post
4async function addComment(postId: string, userId: string, text: string) {
5  const commentsRef = collection(db, 'posts', postId, 'comments')
6  return addDoc(commentsRef, {
7    userId,
8    text,
9    createdAt: serverTimestamp(),
10  })
11}
12
13// Get all comments for a post, ordered by time
14async function getComments(postId: string) {
15  const q = query(
16    collection(db, 'posts', postId, 'comments'),
17    orderBy('createdAt', 'asc')
18  )
19  const snapshot = await getDocs(q)
20  return snapshot.docs.map((doc) => ({ id: doc.id, ...doc.data() }))
21}

1import { doc, updateDoc, arrayUnion, arrayRemove, getDocs, query, where, collection } from 'firebase/firestore'
2
3// Add user to a group
4async function joinGroup(userId: string, groupId: string) {
5  // Add groupId to user's groups array
6  await updateDoc(doc(db, 'users', userId), {
7    groups: arrayUnion(groupId),
8  })
9  // Add userId to group's members array
10  await updateDoc(doc(db, 'groups', groupId), {
11    members: arrayUnion(userId),
12  })
13}
14
15// Find all users in a group
16async function getGroupMembers(groupId: string) {
17  const q = query(
18    collection(db, 'users'),
19    where('groups', 'array-contains', groupId)
20  )
21  const snapshot = await getDocs(q)
22  return snapshot.docs.map((doc) => ({ id: doc.id, ...doc.data() }))
23}
24
25// Remove user from group
26async function leaveGroup(userId: string, groupId: string) {
27  await updateDoc(doc(db, 'users', userId), {
28    groups: arrayRemove(groupId),
29  })
30  await updateDoc(doc(db, 'groups', groupId), {
31    members: arrayRemove(userId),
32  })
33}

1import { addDoc, deleteDoc, getDocs, query, where, collection, doc, getDoc } from 'firebase/firestore'
2
3// Junction collection: group_memberships
4// Each doc: { userId, groupId, role, joinedAt }
5
6async function addMembership(userId: string, groupId: string, role: string) {
7  return addDoc(collection(db, 'group_memberships'), {
8    userId,
9    groupId,
10    role,
11    joinedAt: serverTimestamp(),
12  })
13}
14
15// Find all groups for a user
16async function getUserGroups(userId: string) {
17  const q = query(
18    collection(db, 'group_memberships'),
19    where('userId', '==', userId)
20  )
21  const snapshot = await getDocs(q)
22  const memberships = snapshot.docs.map((d) => d.data())
23
24  // Fetch full group documents
25  const groups = await Promise.all(
26    memberships.map(async (m) => {
27      const groupDoc = await getDoc(doc(db, 'groups', m.groupId))
28      return { id: groupDoc.id, ...groupDoc.data(), role: m.role }
29    })
30  )
31  return groups
32}

1import { doc, setDoc, serverTimestamp } from 'firebase/firestore'
2
3// Store denormalized author data on the post
4async function createPost(userId: string, title: string, body: string) {
5  const userDoc = await getDoc(doc(db, 'users', userId))
6  const userData = userDoc.data()!
7
8  return setDoc(doc(collection(db, 'posts')), {
9    title,
10    body,
11    authorId: userId,
12    // Denormalized copies — avoids reading users collection on every post view
13    authorName: userData.displayName,
14    authorAvatar: userData.photoURL,
15    createdAt: serverTimestamp(),
16  })
17}
18
19// When the user updates their profile, update all their posts too
20// Best done in a Cloud Function trigger on users/{userId}
21// onDocumentUpdated: query posts where authorId == userId,
22// batch update authorName and authorAvatar

1import { collectionGroup, query, where, getDocs, orderBy } from 'firebase/firestore'
2
3// Find all comments by a user across ALL posts
4async function getUserComments(userId: string) {
5  const q = query(
6    collectionGroup(db, 'comments'),
7    where('userId', '==', userId),
8    orderBy('createdAt', 'desc')
9  )
10  const snapshot = await getDocs(q)
11  return snapshot.docs.map((doc) => ({
12    id: doc.id,
13    path: doc.ref.path,  // e.g., 'posts/post-1/comments/comment-1'
14    ...doc.data(),
15  }))
16}
17
18// Security rules for collection group queries must be at the
19// collection group level:
20// match /{path=**}/comments/{commentId} {
21//   allow read: if request.auth != null;
22// }