diff --git a/src/nix/build.cc b/src/nix/build.cc
index 5a3d9d31a..ef6b48969 100644
--- a/src/nix/build.cc
+++ b/src/nix/build.cc
@@ -10,7 +10,7 @@ struct CmdBuild : MixDryRun, InstallablesCommand
 {
     Path outLink = "result";
 
-    bool updateLock = true;
+    bool update = true;
 
     CmdBuild()
     {
@@ -29,7 +29,7 @@ struct CmdBuild : MixDryRun, InstallablesCommand
         mkFlag()
             .longName("no-update")
             .description("don't update the lock file")
-            .set(&updateLock, false);
+            .set(&update, false);
     }
 
     std::string name() override
@@ -77,11 +77,12 @@ struct CmdBuild : MixDryRun, InstallablesCommand
                     }
         }
 
-        // FlakeUri flakeUri = "";
-        // if(updateLock)
-        //     for (uint i = 0; i < installables.size(); i++)
-        //         // if (auto flakeUri = installableToFlakeUri)
-        //             updateLockFile(*evalState, flakeUri);
+        if (update)
+            for (auto installable : installables) {
+                auto flakeUri = installable->installableToFlakeUri();
+                if (flakeUri)
+                    updateLockFile(*evalState, *flakeUri);
+            }
     }
 };
 
diff --git a/src/nix/command.hh b/src/nix/command.hh
index 56e1e6f34..a5ae56fb9 100644
--- a/src/nix/command.hh
+++ b/src/nix/command.hh
@@ -3,6 +3,7 @@
 #include "args.hh"
 #include "primops/flake.hh"
 #include "common-eval-args.hh"
+#include <optional>
 
 namespace nix {
 
@@ -66,6 +67,11 @@ struct Installable
 
     Buildable toBuildable();
 
+    virtual std::optional<std::string> installableToFlakeUri()
+    {
+        return std::nullopt;
+    }
+
     virtual Value * toValue(EvalState & state)
     {
         throw Error("argument '%s' cannot be evaluated", what());
diff --git a/src/nix/installables.cc b/src/nix/installables.cc
index 13a68a797..fc2c34861 100644
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@@ -175,6 +175,14 @@ struct InstallableFlake : InstallableValue
         state.forceValue(*v);
         return v;
     }
+
+    std::optional<std::string> installableToFlakeUri() override
+    {
+        if (std::get_if<FlakeRef::IsPath>(&flakeRef.data))
+            return flakeRef.to_string();
+        else
+            return std::nullopt;
+    }
 };
 
 // FIXME: extend