Prioritize previewable over plain README files

app/models/tree.rb

@@ -17,12 +17,20 @@ class Tree
   def readme
     return @readme if defined?(@readme)
-    # Take the first previewable readme, or return nil if none is available or
-    # we can't preview any of them
-    readme_tree = blobs.find do |blob|
-      blob.readme? && (previewable?(blob.name) || plain?(blob.name))
+    available_readmes = blobs.select(&:readme?)
+
+    previewable_readmes = available_readmes.select do |blob|
+      previewable?(blob.name)
+    end
+
+    plain_readmes = available_readmes.select do |blob|
+      plain?(blob.name)
     end
+    # Prioritize previewable over plain readmes
+    readme_tree = previewable_readmes.first || plain_readmes.first
+
+    # Return if we can't preview any of them
     if readme_tree.nil?
       return @readme = nil
     end

spec/models/tree_spec.rb

+require 'spec_helper'
+
+describe Tree, models: true do
+  let(:repository) { create(:project).repository }
+  let(:sha) { repository.root_ref }
+
+  subject { described_class.new(repository, '54fcc214') }
+
+  describe '#readme' do
+    class FakeBlob
+      attr_reader :name
+
+      def initialize(name)
+        @name = name
+      end
+
+      def readme?
+        name =~ /^readme/i
+      end
+    end
+
+    it 'returns nil when repository does not contains a README file' do
+      files = [FakeBlob.new('file'), FakeBlob.new('license'), FakeBlob.new('copying')]
+      expect(subject).to receive(:blobs).and_return(files)
+
+      expect(subject.readme).to eq nil
+    end
+
+    it 'returns nil when repository does not contains a previewable README file' do
+      files = [FakeBlob.new('file'), FakeBlob.new('README.pages'), FakeBlob.new('README.png')]
+      expect(subject).to receive(:blobs).and_return(files)
+
+      expect(subject.readme).to eq nil
+    end
+
+    it 'returns README when repository contains a previewable README file' do
+      files = [FakeBlob.new('README.png'), FakeBlob.new('README'), FakeBlob.new('file')]
+      expect(subject).to receive(:blobs).and_return(files)
+
+      expect(subject.readme.name).to eq 'README'
+    end
+
+    it 'returns first previewable README when repository contains more than one' do
+      files = [FakeBlob.new('file'), FakeBlob.new('README.md'), FakeBlob.new('README.asciidoc')]
+      expect(subject).to receive(:blobs).and_return(files)
+
+      expect(subject.readme.name).to eq 'README.md'
+    end
+
+    it 'returns first plain text README when repository contains more than one' do
+      files = [FakeBlob.new('file'), FakeBlob.new('README'), FakeBlob.new('README.txt')]
+      expect(subject).to receive(:blobs).and_return(files)
+
+      expect(subject.readme.name).to eq 'README'
+    end
+
+    it 'prioritizes previewable README file over one in plain text' do
+      files = [FakeBlob.new('file'), FakeBlob.new('README'), FakeBlob.new('README.md')]
+      expect(subject).to receive(:blobs).and_return(files)
+
+      expect(subject.readme.name).to eq 'README.md'
+    end
+  end
+end