diff --git a/SignalServiceKit/src/Storage/Database/GRDBSchemaMigrator.swift b/SignalServiceKit/src/Storage/Database/GRDBSchemaMigrator.swift
index fd4b49fe8b..1ca83b8300 100644
--- a/SignalServiceKit/src/Storage/Database/GRDBSchemaMigrator.swift
+++ b/SignalServiceKit/src/Storage/Database/GRDBSchemaMigrator.swift
@@ -19,6 +19,8 @@ public class GRDBSchemaMigrator: NSObject {
         } else {
             try! newUserMigrator.migrate(grdbStorage.pool)
         }
+
+        SSKPreferences.markGRDBSchemaAsLatest()
     }
 
     private var hasCreatedInitialSchema: Bool {
@@ -44,8 +46,15 @@ public class GRDBSchemaMigrator: NSObject {
         case signalAccount_add_contactAvatars
         case signalAccount_add_contactAvatars_indices
         case jobRecords_add_attachmentId
+
+        // NOTE: Every time we add a migration id, consider
+        // incrementing grdbSchemaVersionLatest.
+        // We only need to do this for breaking changes.
     }
 
+    public static let grdbSchemaVersionDefault: UInt = 0
+    public static let grdbSchemaVersionLatest: UInt = 1
+
     // An optimization for new users, we have the first migration import the latest schema
     // and mark any other migrations as "already run".
     private lazy var newUserMigrator: DatabaseMigrator = {
diff --git a/SignalServiceKit/src/Storage/StorageCoordinator.m b/SignalServiceKit/src/Storage/StorageCoordinator.m
index f060405ad2..4d7e9e7f0f 100644
--- a/SignalServiceKit/src/Storage/StorageCoordinator.m
+++ b/SignalServiceKit/src/Storage/StorageCoordinator.m
@@ -113,10 +113,14 @@ NSString *NSStringForDataStore(DataStore value)
         // Allow developers to do this, but not QA, internal,
         // public beta or production.
         !SSKFeatureFlags.canRevertToYDB) {
+        OWSFailDebug(@"Reverting to YDB.");
         return YES;
     }
 
-    // TODO: also return true if unknown GRDB version.
+    if (SSKPreferences.hasUnknownGRDBSchema) {
+        OWSFailDebug(@"Unknown GRDB schema.");
+        return YES;
+    }
 
     return NO;
 }
diff --git a/SignalServiceKit/src/Util/SSKPreferences.swift b/SignalServiceKit/src/Util/SSKPreferences.swift
index 2449542fe6..43a94ff11b 100644
--- a/SignalServiceKit/src/Util/SSKPreferences.swift
+++ b/SignalServiceKit/src/Util/SSKPreferences.swift
@@ -107,4 +107,52 @@ public class SSKPreferences: NSObject {
         appUserDefaults.set(value, forKey: didEverUseYdbKey)
         appUserDefaults.synchronize()
     }
+
+    // MARK: -
+
+    public class var grdbSchemaVersionDefault: UInt {
+        return GRDBSchemaMigrator.grdbSchemaVersionDefault
+    }
+    public class var grdbSchemaVersionLatest: UInt {
+        return GRDBSchemaMigrator.grdbSchemaVersionLatest
+    }
+
+    private static let grdbSchemaVersionKey = "grdbSchemaVersion"
+
+    private static func grdbSchemaVersion() -> UInt {
+        let appUserDefaults = CurrentAppContext().appUserDefaults()
+        guard let preference = appUserDefaults.object(forKey: grdbSchemaVersionKey) as? NSNumber else {
+            return grdbSchemaVersionDefault
+        }
+        return preference.uintValue
+    }
+
+    private static func setGrdbSchemaVersion(_ value: UInt) {
+        let lastKnownGrdbSchemaVersion = grdbSchemaVersion()
+        guard value != lastKnownGrdbSchemaVersion else {
+            return
+        }
+        guard value > lastKnownGrdbSchemaVersion else {
+            owsFailDebug("Reverting to earlier schema version: \(value)")
+            return
+        }
+        Logger.info("Updating schema version: \(lastKnownGrdbSchemaVersion) -> \(value)")
+        let appUserDefaults = CurrentAppContext().appUserDefaults()
+        appUserDefaults.set(value, forKey: grdbSchemaVersionKey)
+        appUserDefaults.synchronize()
+    }
+
+    @objc
+    public static func markGRDBSchemaAsLatest() {
+        setGrdbSchemaVersion(grdbSchemaVersionLatest)
+    }
+
+    @objc
+    public static func hasUnknownGRDBSchema() -> Bool {
+        guard grdbSchemaVersion() <= grdbSchemaVersionLatest else {
+            owsFailDebug("grdbSchemaVersion: \(grdbSchemaVersion()), grdbSchemaVersionLatest: \(grdbSchemaVersionLatest)")
+            return true
+        }
+        return false
+    }
 }