add sort by function

lib/index.js

@@ -41,7 +41,7 @@ function plugin(opts){
     });
 
     /**
-     * Sort the collection.
+     * Sort the collections.
      */
 
     keys.forEach(function(key){
@@ -49,6 +49,10 @@ function plugin(opts){
       var settings = opts[key];
       var sort = settings.sortBy || 'date';
       var col = metadata[key];
+
+      if ('function' == typeof sort) {
+        col.sort(sort);
+      } else {
         col.sort(function(a, b){
           a = a[sort];
           b = b[sort];
@@ -59,6 +63,8 @@ function plugin(opts){
           if (a > b) return 1;
           return 0;
         });
+      }
+
       if (settings.reverse) col.reverse();
     });
 

test/fixtures/sort-function/build/one.md

@@ -0,0 +1 @@
+one

test/fixtures/sort-function/build/three.md

@@ -0,0 +1 @@
+three

test/fixtures/sort-function/build/two.md

@@ -0,0 +1 @@
+two

test/fixtures/sort-function/src/one.md

@@ -0,0 +1,6 @@
+---
+collection: articles
+title: Alpha
+---
+
+one

test/fixtures/sort-function/src/three.md

@@ -0,0 +1,6 @@
+---
+collection: articles
+title: Gamma
+---
+
+three

test/fixtures/sort-function/src/two.md

@@ -0,0 +1,6 @@
+---
+collection: articles
+title: Beta
+---
+
+two

test/index.js

@@ -72,6 +72,26 @@ describe('metalsmith-collections', function(){
       });
   });
 
+  it('should accept a "sortBy" function', function(done){
+    var metalsmith = Metalsmith('test/fixtures/sort');
+    metalsmith
+      .use(collections({ articles: { sortBy: sort }}))
+      .build(function(err){
+        if (err) return done(err);
+        var articles = metalsmith.metadata().articles;
+        assert.equal('Gamma', articles[0].title);
+        assert.equal('Beta', articles[1].title);
+        assert.equal('Alpha', articles[2].title);
+        done();
+      });
+
+    function sort(a, b){
+      a = a.title.slice(1);
+      b = b.title.slice(1);
+      return a > b ? 1 : -1;
+    }
+  });
+
   it('should accept a "reverse" option', function(done){
     var metalsmith = Metalsmith('test/fixtures/sort');
     metalsmith