I have a problem with finding the best approach for forms that contain deeper relationships.

For example I have a form for creating an invoice. While user is filling particular fields he may have to chose a client that is going to get billed. If the client does not exist he should be able to create one. This client might have some relationships to other data for banking details and so on.

There was one application I saw that had for each auto-complete field an extra button to create a new item. When user clicked on "create new item" new modal window popped up.

What is the best approach for such use case? Are there any alternatives to multiple level modal boxes containing such forms?

I have a problem with finding the best approach for forms that contain deeper relationships.

For example I have a form for creating an invoice. While the user is filling particular fields he may have to chose a client that is going to get billed. If the client does not exist, he should be able to create one. This client might have some relationships to other data for banking details and so on.

There was one application I saw that had, for each auto-complete field, an extra button to create a new item. When user clicked on "create new item" a new modal window popped up.

What is the best approach for such use case? Are there any alternatives to multiple level modal boxes containing such forms?