Raise DocumentNotFound instead of NoMethodErros:

- When passing ids to nested attributes and no docs exist, grabbing the
  first class could throw a NoMethodError getting the object id state.
- Fixes #1608.

CHANGELOG.md

@@ -54,6 +54,11 @@ For instructions on upgrading to newer versions, visit
 
 ## 2.4.2 \[ In Development \] \[ Branch: 2.4.0-stable \]
 
+### Resolved Issues
+
+* \#1608 Guard against no method errors when passing ids in nested attributes
+  and the documents do not exist.
+
 * \#1605 Remove deprecation warning on rescue responses, Rails 3.2
 
 * \#1602 Preserve structure of $and and $or queries when typecasting.

lib/mongoid/relations/builders/nested_attributes/many.rb

@@ -92,7 +92,9 @@ def over_limit?(attributes)
           def process(parent, attrs)
             return if reject?(parent, attrs)
             if id = attrs.extract_id
-              doc = existing.find(convert_id(existing.first.class, id))
+              first = existing.first
+              converted = first ? convert_id(first.class, id) : id
+              doc = existing.find(converted)
               if destroyable?(attrs)
                 existing.delete(doc)
                 doc.destroy unless doc.embedded?

spec/mongoid/nested_attributes_spec.rb

@@ -2651,6 +2651,22 @@
                   person.posts(true).size.should == 2
                 end
               end
+
+              context "when there are no documents" do
+
+                before do
+                  person.posts.clear
+                end
+
+                it "raises a document not found error" do
+                  expect {
+                    person.posts_attributes =
+                      { "0" =>
+                        { "id" => BSON::ObjectId.new.to_s, "title" => "Rogue" }
+                      }
+                  }.to raise_error(Mongoid::Errors::DocumentNotFound)
+                end
+              end
             end
 
             context "when the ids do not match" do