Handle named blueprints without a master better.

This raises a more descriptive error, and mentions the need for a master
blueprint in the docs.

Note that the docs refer to a master blueprint as a "default blueprint".
I should unify this language.

README.markdown

@@ -187,6 +187,8 @@ will use the `:admin` blueprint.
 
 Named blueprints call the default blueprint to set any attributes not specifically provided, so in this example the `email` attribute will still be generated even for an admin user.
 
+You must define a default blueprint for any class that has a named blueprint, even if the default blueprint is empty.
+
 
 ### Belongs\_to Associations
 

lib/machinist.rb

@@ -11,7 +11,13 @@ def self.run(adapter, object, *args)
       named_blueprint = object.class.blueprint(args.shift) if args.first.is_a?(Symbol)
       attributes      = args.pop || {}
 
-      raise "No blueprint for class #{object.class}" if blueprint.nil?
+      if blueprint.nil?
+        if named_blueprint
+          raise "Can't construct an object from a named blueprint without a default blueprint for class #{object.class}"
+        else
+          raise "No blueprint for class #{object.class}"
+        end
+      end
 
       lathe = self.new(adapter, object, attributes)
       lathe.instance_eval(&named_blueprint) if named_blueprint

spec/machinist_spec.rb

@@ -29,7 +29,7 @@ class Son < Dad
     end
 
     it "should raise for make on a class with no blueprint" do
-      lambda { Person.make }.should raise_error(RuntimeError)
+      lambda { Person.make }.should raise_error(RuntimeError, "No blueprint for class MachinistSpecs::Person")
     end
 
     it "should set an attribute on the constructed object from a constant in the blueprint" do
@@ -120,7 +120,7 @@ class Son < Dad
         end
         @person = Person.make(:admin)
       end
-
+      
       it "should override an attribute from the parent blueprint in the child blueprint" do
         @person.admin.should == true
       end
@@ -138,6 +138,13 @@ class Son < Dad
         Person.blueprint(:bar) { }
         Person.named_blueprints.to_set.should == [:admin, :foo, :bar].to_set
       end
+
+      it "should raise a sensible error when calling a named blueprint with no master" do
+        Post.blueprint(:foo) { }
+        lambda { Post.make(:foo) }.should raise_error(
+          RuntimeError, "Can't construct an object from a named blueprint without a default blueprint for class MachinistSpecs::Post"
+        )
+      end
     end
 
     describe "blueprint inheritance" do