Add group_by liquid filter

app/concerns/liquid_interpolatable.rb

@@ -283,6 +283,27 @@ def as_object(object)
       throw :as_object, object.as_json
     end
 
+    # Group an array of items by a property
+    #
+    # Example usage:
+    #
+    # {% assign posts_by_author = site.posts | group_by: "author" %}
+    # {% for author in posts_by_author %}
+    #   <dt>{{author.name}}</dt>
+    #   {% for post in author.items %}
+    #   <dd><a href="{{post.url}}">{{post.title}}</a></dd>
+    #   {% endfor %}
+    # {% endfor %}
+    def group_by(input, property)
+      if input.respond_to?(:group_by)
+        input.group_by { |item| item[property] }.map do |value, items|
+          { 'name' => value, 'items' => items }
+        end
+      else
+        input
+      end
+    end
+
     private
 
     def logger

spec/concerns/liquid_interpolatable_spec.rb

@@ -86,19 +86,19 @@ def @filter.to_xpath_roundtrip(string)
       @agent.interpolation_context['s'] = 'http://example.com/dir/1?q=test'
     end
 
-    it 'should parse an abosule URI' do
+    it 'should parse an absolute URI' do
       expect(@filter.to_uri('http://example.net/index.html', 'http://example.com/dir/1')).to eq(URI('http://example.net/index.html'))
     end
 
-    it 'should parse an abosule URI with a base URI specified' do
+    it 'should parse an absolute URI with a base URI specified' do
       expect(@filter.to_uri('http://example.net/index.html', 'http://example.com/dir/1')).to eq(URI('http://example.net/index.html'))
     end
 
     it 'should parse a relative URI with a base URI specified' do
       expect(@filter.to_uri('foo/index.html', 'http://example.com/dir/1')).to eq(URI('http://example.com/dir/foo/index.html'))
     end
 
-    it 'should parse an abosule URI with a base URI specified' do
+    it 'should parse an absolute URI with a base URI specified' do
       expect(@filter.to_uri('http://example.net/index.html', 'http://example.com/dir/1')).to eq(URI('http://example.net/index.html'))
     end
 
@@ -394,4 +394,40 @@ def ensure_safety(obj)
       expect(agent.interpolated['template']).to eq('38b98bc2625a8cac33369f6204e784482be5e172b242699406270856a841d1ec')
     end
   end
+
+  describe 'group_by' do
+    let(:events) do
+      [
+        { "date" => "2019-07-30", "type" => "Snap" },
+        { "date" => "2019-07-30", "type" => "Crackle" },
+        { "date" => "2019-07-29", "type" => "Pop" },
+        { "date" => "2019-07-29", "type" => "Bam" },
+        { "date" => "2019-07-29", "type" => "Pow" },
+      ]
+    end
+
+    it "should group an enumerable by the given attribute" do
+      expect(@filter.group_by(events, "date")).to eq(
+        [
+          {
+            "name" => "2019-07-30", "items" => [
+              { "date" => "2019-07-30", "type" => "Snap" },
+              { "date" => "2019-07-30", "type" => "Crackle" }
+            ]
+          },
+          {
+            "name" => "2019-07-29", "items" => [
+              { "date" => "2019-07-29", "type" => "Pop" },
+              { "date" => "2019-07-29", "type" => "Bam" },
+              { "date" => "2019-07-29", "type" => "Pow" }
+            ]
+          }
+        ]
+      )
+    end
+
+    it "should leave non-groupables alone" do
+      expect(@filter.group_by("some string", "anything")).to eq("some string")
+    end
+  end
 end