fix: resolve source graph in separate query

src/main/kotlin/org/openmbee/flexo/mms/routes/gsp/ModelLoad.kt

@@ -217,6 +217,80 @@ fun Route.loadModel() {
 
             // compute the delta
             run {
+                val selectQueryString = """
+                    select distinct ?srcGraph {
+                        graph mor-graph:Metadata {
+                            # select the latest commit from the current named ref
+                            ?srcRef mms:commit ?srcCommit .
+
+                            # get the latest snapshot associated with the source commit
+                            ?srcCommit ^mms:commit/mms:snapshot ?srcSnapshot .
+                            {
+                                # prefer the model snapshot
+                                ?srcSnapshot a mms:Model ;
+                                    mms:graph ?srcGraph  .
+                            } union {
+                                # settle for staging...
+                                ?srcSnapshot a mms:Staging ;
+                                    mms:graph ?srcGraph .
+
+                                # ...if model is not available
+                                filter not exists {
+                                    ?srcCommit ^mms:commit/mms:snapshot/a mms:Model .
+                                }
+                            }
+                        }
+
+                        bind(
+                            sha256(
+                                concat(str(?dstCommit), "\n", str(?srcCommit))
+                            ) as ?diffId
+                        )
+
+                        bind(
+                            iri(
+                                concat(str(?dstCommit), "/diffs/", ?diffId)
+                            ) as ?diff
+                        )
+
+                        bind(
+                            iri(
+                                concat(str(mor-graph:), "Diff.Ins.", ?diffId)
+                            ) as ?insGraph
+                        )
+
+                        bind(
+                            iri(
+                                concat(str(mor-graph:), "Diff.Del.", ?diffId)
+                            ) as ?delGraph
+                        )
+                    }
+                """.trimIndent()
+
+                val selectResponseText = executeSparqlSelectOrAsk(selectQueryString) {
+                    prefixes(prefixes)
+
+                    iri(
+                        // use current branch as ref source
+                        "srcRef" to prefixes["morb"]!!,
+
+                        // set dst graph
+                        "dstGraph" to loadGraphUri,
+
+                        // set dst commit (this commit)
+                        "dstCommit" to prefixes["morc"]!!,
+                    )
+                }
+
+                // parse the JSON response
+                val bindings = Json.parseToJsonElement(selectResponseText).jsonObject["results"]!!.jsonObject["bindings"]!!.jsonArray
+
+                if(bindings.size != 1) {
+                    throw ServerBugException("Failed to resolve source graph")
+                }
+
+                val sourceGraphIri = bindings[0].jsonObject["srcGraph"]!!.jsonObject["value"]!!.jsonPrimitive.content
+
                 val updateString = genDiffUpdate("", localConditions, """
                     graph mor-graph:Metadata {
                         # select the latest commit from the current named ref
@@ -253,6 +327,9 @@ fun Route.loadModel() {
 
                         // set dst commit (this commit)
                         "dstCommit" to prefixes["morc"]!!,
+
+                        // use explicit srcGraph
+                        "srcGraph" to sourceGraphIri,
                     )
                 }
             }