Fix: Migrate user structural data to linked data

coscine/issues#2081

src/SQL2Linked/Implementations/UserStructuralData.cs

 using Coscine.Database.DataModel;
 using Coscine.Database.Models;
+using Coscine.Metadata;
 using VDS.RDF;
 
 namespace SQL2Linked.Implementations
 {
     public class UserStructuralData : StructuralData<User, UserModel>
     {
+        public readonly string UserUrlPrefix = "https://purl.org/coscine/users";
+        public readonly Uri RdfType = new("http://www.w3.org/1999/02/22-rdf-syntax-ns#type");
+        public readonly Uri FoafPerson = new("http://xmlns.com/foaf/0.1/Person");
+
         public override IEnumerable<IGraph> ConvertToLinkedData(IEnumerable<User> entries)
         {
-            // ToDo: Implement
-            throw new NotImplementedException();
+            var graphs = new List<IGraph>();
+
+            foreach (var entry in entries)
+            {
+                var userGraphName = $"{UserUrlPrefix}/{entry.Id}";
+                var graph = RdfStoreConnector.GetGraph(userGraphName);
+
+                // check if a triple with a foaf:Person already exists in the user graph
+                var getTriples = graph.GetTriplesWithObject(FoafPerson);
+
+                if (!getTriples.Any())
+                {
+                    // check if a user graph already exists
+                    var exists = RdfStoreConnector.HasGraph(userGraphName);
+
+                    if (!exists)
+                    {
+                        RdfStoreConnector.CreateNamedGraph(userGraphName);
+                    }
+
+                    graph.Assert(
+                        new Triple(
+                            graph.CreateBlankNode(),
+                            graph.CreateUriNode(RdfType),
+                            graph.CreateUriNode(FoafPerson)
+                        )
+                    );
+
+                    graphs.Add(graph);
+
+                    Console.WriteLine($"Will migrate user '{entry.DisplayName}' with id '{entry.Id}'.");
+                }
+                else
+                {
+                    Console.WriteLine($"Will NOT migrate user '{entry.DisplayName}' with id '{entry.Id}'.");
+                } 
+            }
+            return graphs;
         }
     }
-}
\ No newline at end of file
+}