Add RDF.Graph.take/3

CHANGELOG.md

@@ -11,6 +11,8 @@ This project adheres to [Semantic Versioning](http://semver.org/) and
 
 - `RDF.Description.take/2` creates a description from another one by limiting 
   its statements to a set of predicates
+- `RDF.Graph.take/3` creates a graph from another one by limiting 
+  its statements to a set of subjects and optionally also a set of predicates
 - Mix formatter configuration for using `defvocab` without parens 
 
 

lib/rdf/description.ex

@@ -367,6 +367,15 @@ defmodule RDF.Description do
     |> List.first
   end
 
+
+#  def update(description = %RDF.Description{}, predicate, initial \\ [], fun) do
+#    triple_predicate = coerce_predicate(predicate)
+#    description.predicates
+#    |> Map.update(triple_predicate, initial, fn objects ->
+#    end)
+#  end
+
+
   @doc """
   Gets and updates the objects of the given predicate of a Description, in a single pass.
 
@@ -620,7 +629,13 @@ defmodule RDF.Description do
   Creates a description from another one by limiting its statements to those using one of the given `predicates`.
 
   If `predicates` contains properties that are not used in the `description`, they're simply ignored.
+
+  If `nil` is passed, the description is left untouched.
   """
+  def take(description, predicates)
+
+  def take(%RDF.Description{} = description, nil), do: description
+
   def take(%RDF.Description{predications: predications} = description, predicates) do
     predicates = Enum.map(predicates, &(coerce_predicate/1))
     %RDF.Description{description | predications: Map.take(predications, predicates)}

lib/rdf/graph.ex

@@ -686,6 +686,33 @@ defmodule RDF.Graph do
     end
   end
 
+  @doc """
+  Creates a graph from another one by limiting its statements to those using one of the given `subjects`.
+
+  If `subjects` contains IRIs that are not used in the `graph`, they're simply ignored.
+
+  The optional `properties` argument allows to limit also properties of the subject descriptions.
+
+  If `nil` is passed as the `subjects`, the subjects will not be limited.
+  """
+  def take(graph, subjects, properties \\ nil)
+
+  def take(%RDF.Graph{} = graph, nil, nil), do: graph
+
+  def take(%RDF.Graph{descriptions: descriptions} = graph, subjects, nil) do
+    subjects = Enum.map(subjects, &(coerce_subject/1))
+    %RDF.Graph{graph | descriptions: Map.take(descriptions, subjects)}
+  end
+
+  def take(%RDF.Graph{} = graph, subjects, properties) do
+    graph = take(graph, subjects, nil)
+    %RDF.Graph{graph | 
+      descriptions: Map.new(graph.descriptions, fn {subject, description} ->
+        {subject, Description.take(description, properties)}
+      end)
+    }
+  end
+
 
   @doc """
   Checks if two `RDF.Graph`s are equal.

test/unit/description_test.exs

@@ -357,10 +357,23 @@ defmodule RDF.DescriptionTest do
              %{p: ["Foo"]}
   end
 
-  test "take/2" do
-    assert Description.new([{EX.S, EX.p1, EX.O1}, {EX.S, EX.p2, EX.O2}])
-           |> Description.take([EX.p2, EX.p3]) ==
-             Description.new({EX.S, EX.p2, EX.O2})
+  describe "take/2" do
+    test "with a non-empty property list" do
+      assert Description.new([{EX.S, EX.p1, EX.O1}, {EX.S, EX.p2, EX.O2}])
+             |> Description.take([EX.p2, EX.p3]) ==
+               Description.new({EX.S, EX.p2, EX.O2})
+    end
+
+    test "with an empty property list" do
+      assert Description.new([{EX.S, EX.p1, EX.O1}, {EX.S, EX.p2, EX.O2}])
+             |> Description.take([]) == Description.new(EX.S)
+    end
+
+    test "with nil" do
+      assert Description.new([{EX.S, EX.p1, EX.O1}, {EX.S, EX.p2, EX.O2}])
+             |> Description.take(nil) ==
+               Description.new([{EX.S, EX.p1, EX.O1}, {EX.S, EX.p2, EX.O2}])
+    end
   end
 
   test "equal/2" do

test/unit/graph_test.exs

@@ -479,6 +479,48 @@ defmodule RDF.GraphTest do
              }
   end
 
+  describe "take/2" do
+    test "with a non-empty subject list" do
+      assert Graph.new([{EX.s1, EX.p, EX.o1}, {EX.s2, EX.p, EX.o2}])
+             |> Graph.take([EX.s2, EX.s3]) ==
+               Graph.new([{EX.s2, EX.p, EX.o2}])
+    end
+
+    test "with an empty subject list" do
+      assert Graph.new([{EX.s1, EX.p, EX.o1}, {EX.s2, EX.p, EX.o2}])
+             |> Graph.take([]) == Graph.new()
+    end
+
+    test "with nil" do
+      assert Graph.new([{EX.s1, EX.p, EX.o1}, {EX.s2, EX.p, EX.o2}])
+             |> Graph.take(nil) ==
+               Graph.new([{EX.s1, EX.p, EX.o1}, {EX.s2, EX.p, EX.o2}])
+    end
+  end
+
+  describe "take/3" do
+    test "with non-empty subject and property lists" do
+      assert Graph.new([{EX.s1, EX.p1, EX.o1}, {EX.s1, EX.p2, EX.o1}, {EX.s2, EX.p1, EX.o2}])
+             |> Graph.take([EX.s1, EX.s3], [EX.p2]) ==
+               Graph.new([{EX.s1, EX.p2, EX.o1}])
+    end
+
+    test "with an empty subject list" do
+      assert Graph.new(
+               [
+                 {EX.s1, EX.p1, EX.o1},
+                 {EX.s1, EX.p2, EX.o1},
+                 {EX.s2, EX.p1, EX.o2}
+               ], name: EX.Graph)
+             |> Graph.take([], [EX.p1]) == Graph.new(name: EX.Graph)
+    end
+
+    test "with nil" do
+      assert Graph.new([{EX.s1, EX.p1, EX.o1}, {EX.s1, EX.p2, EX.o1}, {EX.s2, EX.p1, EX.o2}])
+             |> Graph.take(nil, [EX.p1]) ==
+               Graph.new([{EX.s1, EX.p1, EX.o1}, {EX.s2, EX.p1, EX.o2}])
+    end
+  end
 
   test "equal/2" do
     assert Graph.new({EX.S, EX.p, EX.O}) |> Graph.equal?(Graph.new({EX.S, EX.p, EX.O}))