From ca724d030068442664673d3b5ba17bff8d01dda2 Mon Sep 17 00:00:00 2001
From: Marcel Otto <marcelotto@gmx.de>
Date: Thu, 31 Mar 2022 00:13:57 +0200
Subject: [PATCH] Fix graph addition of triples with an empty object list

---
 CHANGELOG.md                   |  6 ++++
 lib/rdf/graph.ex               | 54 +++++++++++++++-------------------
 test/unit/description_test.exs |  5 ++++
 test/unit/graph_test.exs       | 10 +++++++
 4 files changed, 44 insertions(+), 31 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 94bed78..4141a52 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,12 @@ This project adheres to [Semantic Versioning](http://semver.org/) and
 - The inspect string of `RDF.Description` now includes the subject separately, so 
   it can be seen also when the description is empty.
 
+### Fixed
+
+- When triples with an empty object list where added to an `RDF.Graph`, it 
+  included empty descriptions, which lead to inconsistent behaviour 
+  (for example it would be counted in `RDF.Graph.subject_count/1`).
+
 
 [Compare v0.11.0...HEAD](https://github.com/rdf-elixir/rdf-ex/compare/v0.11.0...HEAD)
 
diff --git a/lib/rdf/graph.ex b/lib/rdf/graph.ex
index c31e86f..03562f2 100644
--- a/lib/rdf/graph.ex
+++ b/lib/rdf/graph.ex
@@ -18,8 +18,6 @@ defmodule RDF.Graph do
   alias RDF.{Description, IRI, PrefixMap, PropertyMap}
   alias RDF.Star.Statement
 
-  import RDF.Utils
-
   @type graph_description :: %{Statement.subject() => Description.t()}
 
   @type t :: %__MODULE__{
@@ -192,20 +190,21 @@ defmodule RDF.Graph do
   @spec add(t, input, keyword) :: t
   def add(graph, input, opts \\ [])
 
-  def add(%__MODULE__{} = graph, {subject, predications}, opts),
-    do: do_add(graph, RDF.coerce_subject(subject), predications, opts)
-
-  def add(%__MODULE__{} = graph, {subject, _, _} = triple, opts),
-    do: do_add(graph, RDF.coerce_subject(subject), triple, opts)
-
-  def add(graph, {subject, predicate, object, _}, opts),
-    do: add(graph, {subject, predicate, object}, opts)
-
-  def add(%__MODULE__{} = graph, %Description{} = description, opts) do
-    if Description.count(description) > 0 do
-      do_add(graph, description.subject, description, opts)
-    else
+  def add(%__MODULE__{descriptions: descriptions} = graph, %Description{} = description, opts) do
+    if Enum.empty?(description) do
       graph
+    else
+      %__MODULE__{
+        graph
+        | descriptions:
+            Map.update(
+              descriptions,
+              description.subject,
+              description,
+              &Description.add(&1, description, opts)
+            )
+      }
+      |> RDF.Star.Graph.handle_addition_annotations(description, opts)
     end
   end
 
@@ -234,26 +233,19 @@ defmodule RDF.Graph do
     |> Enum.reduce(graph, &add(&2, &1, opts))
   end
 
+  def add(%__MODULE__{} = graph, {subject, predications}, opts),
+    do: add(graph, Description.new(subject, Keyword.put(opts, :init, predications)), opts)
+
+  def add(%__MODULE__{} = graph, {subject, _, _} = triple, opts),
+    do: add(graph, Description.new(subject, Keyword.put(opts, :init, triple)), opts)
+
+  def add(graph, {subject, predicate, object, _}, opts),
+    do: add(graph, {subject, predicate, object}, opts)
+
   def add(graph, input, opts) when is_list(input) or (is_map(input) and not is_struct(input)) do
     Enum.reduce(input, graph, &add(&2, &1, opts))
   end
 
-  defp do_add(%__MODULE__{descriptions: descriptions} = graph, subject, statements, opts) do
-    %__MODULE__{
-      graph
-      | descriptions:
-          lazy_map_update(
-            descriptions,
-            subject,
-            # when new: create and initialize description with statements
-            fn -> Description.new(subject, Keyword.put(opts, :init, statements)) end,
-            # when update: merge statements description
-            fn description -> Description.add(description, statements, opts) end
-          )
-    }
-    |> RDF.Star.Graph.handle_addition_annotations({subject, statements}, opts)
-  end
-
   @doc """
   Adds statements to a `RDF.Graph` overwriting existing statements with the subjects given in the `input` data.
 
diff --git a/test/unit/description_test.exs b/test/unit/description_test.exs
index 08e02ab..ce238d1 100644
--- a/test/unit/description_test.exs
+++ b/test/unit/description_test.exs
@@ -124,6 +124,11 @@ defmodule RDF.DescriptionTest do
       assert description_includes_predication(desc, {EX.p(), iri(EX.O2)})
     end
 
+    test "with a predicate-object tuple and an empty list of objects" do
+      assert Description.add(description(), {EX.p(), []}) ==
+               description()
+    end
+
     test "with a list of predicate-object tuples" do
       desc =
         Description.add(description(), [
diff --git a/test/unit/graph_test.exs b/test/unit/graph_test.exs
index 7c00748..585a3ac 100644
--- a/test/unit/graph_test.exs
+++ b/test/unit/graph_test.exs
@@ -74,6 +74,10 @@ defmodule RDF.GraphTest do
       assert graph_includes_statement?(g, {EX.Subject, EX.predicate(), EX.Object2})
     end
 
+    test "initial triples with an empty object list" do
+      assert Graph.new({EX.Subject, EX.predicate(), []}) == Graph.new()
+    end
+
     test "creating a named graph with an initial description" do
       g =
         Description.new(EX.Subject, init: {EX.predicate(), EX.Object})
@@ -336,6 +340,12 @@ defmodule RDF.GraphTest do
       assert graph_includes_statement?(g, {EX.S2, EX.P2, EX.O2})
     end
 
+    test "empty object list" do
+      assert Graph.add(graph(), {EX.S, EX.P, []}) == graph()
+      graph = Graph.new({EX.S, EX.P, EX.O})
+      assert Graph.add(graph, {EX.S, EX.P, []}) == graph
+    end
+
     test "a mixed list" do
       g =
         Graph.new([{EX.S1, EX.p1(), EX.O1}, {EX.S2, EX.p2(), EX.O2}, {EX.S1, EX.p3(), EX.O3}])