Fix ExternalLoads not copying the underlying connection to its associ…

…ated document (#3926)

CHANGELOG.md

@@ -26,6 +26,8 @@ v4.6
 - Made improvements to `MocoUtilities::createExternalLoadsTableForGait()`: center of pressure values are now set to zero, rather 
   than NaN, when vertical force is zero, and the vertical torque is returned in the torque columns (rather than the sum of the 
   sphere torques) to be consistent with the center of pressure GRF representation.
+- Fixed an issue where a copy of an `OpenSim::Model` containing a `OpenSim::ExternalLoads` could not be
+  finalized (#3926)
 
 v4.5.1
 ======

OpenSim/Simulation/Model/ExternalLoads.cpp

@@ -76,6 +76,12 @@ ExternalLoads::ExternalLoads(const ExternalLoads &otherExternalLoads) :
     ModelComponentSet<ExternalForce>(otherExternalLoads),
     _dataFileName(_dataFileNameProp.getValueStr())
 {
+    // copy the document over, because it's used during `extendFinalizeConnections`
+    // to figure out where the associated motion file (#3926)
+    if (auto* document = otherExternalLoads.getDocument()) {
+        setDocument(std::make_unique<XMLDocument>(*document).release());
+    }
+
     setNull();
 
     // Class Members

OpenSim/Tools/Test/CMakeLists.txt

@@ -12,5 +12,5 @@ OpenSimAddTests(
 
 
 # edge-case: one test ensures that `ExternalLoads` still works - even when it isn't in
-#            the working directory of the test runner
+#            the working directory of the test runner (#3926)
 file(COPY "ExternalLoadsInSubdir" DESTINATION "${CMAKE_CURRENT_BINARY_DIR}")

OpenSim/Tools/Test/testExternalLoads.cpp

@@ -296,6 +296,10 @@ TEST_CASE("ExternalLoads Default Properties")
     model.initSystem();
 }
 
+// related: #3926
+//
+// adding a valid `OpenSim::ExternalLoads` to a model shouldn't result in a model
+// that cannot be copied.
 TEST_CASE("ExternalLoads Can Be Copied")
 {
     OpenSim::Model model{"ExternalLoadsInSubdir/model-in-subdir.osim"};