Added require keys method to raise if not defined

README.md

@@ -163,6 +163,21 @@ SECRET_KEY=YOURSECRETKEYGOESHERE # comment
 SECRET_HASH="something-with-a-#-hash"
 ```
 
+### Required Keys
+
+If a particular configuration value is required but not set, it's appropriate to raise an error.
+
+To require configuration keys:
+
+```ruby
+# config/initializers/dotenv.rb
+
+Dotenv.require_keys("SERVICE_APP_ID", "SERVICE_KEY", "SERVICE_SECRET")
+```
+
+If any of the configuration keys above are not set, your application will raise an error during initialization. This method is preferred because it prevents runtime errors in a production application due to improper configuration.
+
+
 ## Frequently Answered Questions
 
 ### Can I use dotenv in production?

lib/dotenv.rb

@@ -1,5 +1,6 @@
 require "dotenv/parser"
 require "dotenv/environment"
+require "dotenv/missing_keys"
 
 # The top level Dotenv module. The entrypoint for the application logic.
 module Dotenv
@@ -55,6 +56,12 @@ def instrument(name, payload = {}, &block)
     end
   end
 
+  def require_keys(*keys)
+    missing_keys = keys.flatten - ::ENV.keys
+    return if missing_keys.empty?
+    raise MissingKeys, missing_keys
+  end
+
   def ignoring_nonexistent_files
     yield
   rescue Errno::ENOENT

lib/dotenv/missing_keys.rb

@@ -0,0 +1,10 @@
+module Dotenv
+  class Error < StandardError; end
+
+  class MissingKeys < Error # :nodoc:
+    def initialize(keys)
+      key_word = "key#{keys.size > 1 ? 's' : ''}"
+      super("Missing required configuration #{key_word}: #{keys.inspect}")
+    end
+  end
+end

spec/dotenv_spec.rb

@@ -153,6 +153,20 @@
     end
   end
 
+  describe "require keys" do
+    let(:env_files) { [".env", fixture_path("bom.env")] }
+
+    before { Dotenv.load(*env_files) }
+
+    it "raises exception with not defined mandatory ENV keys" do
+      expect { Dotenv.require_keys("BOM", "TEST") }.to \
+        raise_error(
+          Dotenv::MissingKeys,
+          'Missing required configuration key: ["TEST"]'
+        )
+    end
+  end
+
   describe "Unicode" do
     subject { fixture_path("bom.env") }