From 58c577201134c81ab0e8c2e9cc33543210ad2892 Mon Sep 17 00:00:00 2001
From: Marcel Otto <marcelotto@gmx.de>
Date: Tue, 28 Jul 2020 14:45:12 +0200
Subject: [PATCH] Add RDF.Description.subject/1 and
 RDF.Description.change_subject/2

---
 CHANGELOG.md                   |  4 ++++
 lib/rdf/description.ex         | 16 +++++++++++++++-
 test/unit/description_test.exs |  9 +++++++++
 3 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0dee707..8ca2791 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,10 @@ This project adheres to [Semantic Versioning](http://semver.org/) and
 
 ## Unreleased
 
+### Added
+
+- `RDF.Description.subject/1` and `RDF.Description.change_subject/2`
+
 ### Changed
 
 - for consistency reasons the internal `:id` struct field of `RDF.BlankNode` was renamed
diff --git a/lib/rdf/description.ex b/lib/rdf/description.ex
index 2790e7f..7881abc 100644
--- a/lib/rdf/description.ex
+++ b/lib/rdf/description.ex
@@ -50,6 +50,18 @@ defmodule RDF.Description do
   def new({subject, predicate, object}), do: new(subject) |> add({predicate, object})
   def new(subject), do: %__MODULE__{subject: coerce_subject(subject)}
 
+  @doc """
+  Returns the subject IRI or blank node of a description.
+  """
+  def subject(%__MODULE__{} = description), do: description.subject
+
+  @doc """
+  Changes the subject of a description.
+  """
+  def change_subject(%__MODULE__{} = description, new_subject) do
+    %__MODULE__{description | subject: coerce_subject(new_subject)}
+  end
+
   @doc """
   Add statements to a `RDF.Description`.
 
@@ -130,7 +142,9 @@ defmodule RDF.Description do
 
   Note: As it is a destructive function this function is more strict in its handling of
   `RDF.Description`s than `add/3`. The subject of a `RDF.Description` to be put must
-  match. 
+  match. If you want to overwrite existing statements with those from the description of
+  another subject, you'll have to explicitly change the subject with `change_subject/2`
+  first before using `put/3`.
 
   ## Examples
 
diff --git a/test/unit/description_test.exs b/test/unit/description_test.exs
index 1c31fa8..e5db30c 100644
--- a/test/unit/description_test.exs
+++ b/test/unit/description_test.exs
@@ -34,6 +34,15 @@ defmodule RDF.DescriptionTest do
     end
   end
 
+  test "subject/1" do
+    assert Description.subject(description()) == description().subject
+  end
+
+  test "change_subject/2" do
+    assert Description.change_subject(description(), EX.NewSubject).subject ==
+             iri(EX.NewSubject)
+  end
+
   describe "add/3" do
     test "with a triple" do
       assert Description.add(description(), {iri(EX.Subject), EX.predicate(), iri(EX.Object)})