Follow depended gem version by fluent-plugin-generate

lib/fluent/command/plugin_generator.rb

@@ -151,6 +151,36 @@ def plugin_name
     underscore_name
   end
 
+  def gem_file_path
+    File.expand_path(File.join(File.dirname(__FILE__),
+                               "../../../",
+                               "Gemfile"))
+  end
+
+  def lock_file_path
+    File.expand_path(File.join(File.dirname(__FILE__),
+                               "../../../",
+                               "Gemfile.lock"))
+  end
+
+  def locked_gem_version(gem_name)
+    d = Bundler::Definition.build(gem_file_path, lock_file_path, false)
+    d.locked_gems.dependencies[gem_name].requirement.requirements.first.last.version
+  end
+
+  def rake_version
+    locked_gem_version("rake")
+  end
+
+  def test_unit_version
+    locked_gem_version("test-unit")
+  end
+
+  def bundler_version
+    d = Bundler::Definition.build(gem_file_path, lock_file_path, false)
+    d.locked_gems.bundler_version.version
+  end
+
   def class_name
     "#{capitalized_name}#{type.capitalize}"
   end

templates/new_gem/fluent-plugin.gemspec.erb

@@ -20,8 +20,8 @@ Gem::Specification.new do |spec|
   spec.test_files    = test_files
   spec.require_paths = ["lib"]
 
-  spec.add_development_dependency "bundler", "~> 1.14"
-  spec.add_development_dependency "rake", "~> 12.0"
-  spec.add_development_dependency "test-unit", "~> 3.0"
+  spec.add_development_dependency "bundler", "~> <%= bundler_version %>"
+  spec.add_development_dependency "rake", "~> <%= rake_version %>"
+  spec.add_development_dependency "test-unit", "~> <%= test_unit_version %>"
   spec.add_runtime_dependency "fluentd", [">= 0.14.10", "< 2"]
 end