core: add statements from one graph to another

lib/rdf/graph.ex

@@ -120,11 +120,18 @@ defmodule RDF.Graph do
 
   def add(%RDF.Graph{name: name, descriptions: descriptions},
           %Description{subject: subject} = description) do
-    description = if existing_description = descriptions[subject],
-      do:   Description.add(existing_description, description),
-      else: description
     %RDF.Graph{name: name,
-               descriptions: Map.put(descriptions, subject, description)}
+      descriptions:
+        Map.update(descriptions, subject, description, fn current ->
+          current |> Description.add(description)
+        end)
+    }
+  end
+
+  def add(graph, %RDF.Graph{descriptions: descriptions}) do
+    Enum.reduce descriptions, graph, fn ({_, description}, graph) ->
+      add(graph, description)
+    end
   end
 
 

test/unit/graph_test.exs

@@ -143,6 +143,27 @@ defmodule RDF.GraphTest do
       assert Graph.add(g, {EX.Subject, EX.predicate, EX.Object}) == g
     end
 
+    test "a Graph" do
+      g = Graph.add(graph(), Graph.new([
+        {EX.Subject1, EX.predicate1, EX.Object1},
+        {EX.Subject2, EX.predicate2, EX.Object2},
+        {EX.Subject3, EX.predicate3, EX.Object3}
+      ]))
+      assert graph_includes_statement?(g, {EX.Subject1, EX.predicate1, EX.Object1})
+      assert graph_includes_statement?(g, {EX.Subject2, EX.predicate2, EX.Object2})
+      assert graph_includes_statement?(g, {EX.Subject3, EX.predicate3, EX.Object3})
+
+      g = Graph.add(g, Graph.new([
+        {EX.Subject1, EX.predicate1, EX.Object2},
+        {EX.Subject2, EX.predicate4, EX.Object4},
+      ]))
+      assert graph_includes_statement?(g, {EX.Subject1, EX.predicate1, EX.Object1})
+      assert graph_includes_statement?(g, {EX.Subject1, EX.predicate1, EX.Object2})
+      assert graph_includes_statement?(g, {EX.Subject2, EX.predicate2, EX.Object2})
+      assert graph_includes_statement?(g, {EX.Subject2, EX.predicate4, EX.Object4})
+      assert graph_includes_statement?(g, {EX.Subject3, EX.predicate3, EX.Object3})
+    end
+
     test "non-convertible Triple elements are causing an error" do
       assert_raise RDF.InvalidURIError, fn ->
         Graph.add(graph(), {"not a URI", EX.predicate, uri(EX.Object)})