diff --git a/db/migrate/20250806140001_create_inbox_assignment_policies.rb b/db/migrate/20250806140001_create_inbox_assignment_policies.rb
index e80c67679..37fd6e37e 100644
--- a/db/migrate/20250806140001_create_inbox_assignment_policies.rb
+++ b/db/migrate/20250806140001_create_inbox_assignment_policies.rb
@@ -3,7 +3,7 @@
 class CreateInboxAssignmentPolicies < ActiveRecord::Migration[7.1]
   def change
     create_table :inbox_assignment_policies do |t|
-      t.references :inbox, null: false, index: true
+      t.references :inbox, null: false, index: { unique: true }
       t.references :assignment_policy, null: false, index: true
 
       t.timestamps
diff --git a/db/schema.rb b/db/schema.rb
index 43996ac45..4b192ac43 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -762,7 +762,7 @@ ActiveRecord::Schema[7.1].define(version: 2025_08_06_140005) do
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
     t.index ["assignment_policy_id"], name: "index_inbox_assignment_policies_on_assignment_policy_id"
-    t.index ["inbox_id"], name: "index_inbox_assignment_policies_on_inbox_id"
+    t.index ["inbox_id"], name: "index_inbox_assignment_policies_on_inbox_id", unique: true
   end
 
   create_table "inbox_capacity_limits", force: :cascade do |t|