Decrement follow count on unfollow

functions/src/follow-market.ts

@@ -21,7 +21,6 @@ export const addUserToContractFollowers = async (
       id: user.id,
       createdTime: Date.now(),
     })
-  // TODO: decrement for unfollows
   await firestore
     .collection(`contracts`)
     .doc(contract.id)

functions/src/index.ts

@@ -30,6 +30,7 @@ export * from './score-contracts'
 export * from './weekly-markets-emails'
 export * from './reset-betting-streaks'
 export * from './reset-weekly-emails-flag'
+export * from './on-delete-contract-follow'
 
 // v2
 export * from './health'

functions/src/on-delete-contract-follow.ts

@@ -0,0 +1,24 @@
+import * as functions from 'firebase-functions'
+import * as admin from 'firebase-admin'
+import { FieldValue } from 'firebase-admin/firestore'
+
+export const onDeleteContractFollow = functions.firestore
+  .document('contracts/{contractId}/follows/{userId}')
+  .onDelete(async (change, context) => {
+    const { contractId } = context.params as {
+      contractId: string
+    }
+    const firestore = admin.firestore()
+    const contract = await firestore
+      .collection(`contracts`)
+      .doc(contractId)
+      .get()
+    if (!contract.exists) throw new Error('Could not find contract')
+
+    await firestore
+      .collection(`contracts`)
+      .doc(contractId)
+      .update({
+        followerCount: FieldValue.increment(-1),
+      })
+  })