Building complex form with Symfony

I have an app where you can select partials which will be used for generating a full document. Partials are divided into categories. And there are 2 types of partials:

• "invisible" - will be included always when category has been selected
• "selectable" - will be included if both category and this partial has been selected

Example:

• Category: Automotive
  • Invisible partial A.1: Introduction
  • Selectable partial A.2: Comparison table
• Category: IT
  • Invisible partial IT.1: Introduction
  • Selectable partial IT.2: Comparison table

This should be rendered as:

[ ] Automotive
    [ ] Comparison table
[ ] IT
    [ ] Comparison table

Selected partials (both invisibile and selectable) are copied are into a separate doc_partial table as their content is could to be changed. For example if I select them like this:

[x] Automotive
    [x] Comparison table
[x] IT
    [ ] Comparison table

...that means following partials are copied into doc_partial:

• A.1 - because Automotive was selected
• A.2 - because Automotive AND A.2 was selected
• IT.1 - because IT was selected

User can also change document structure later, i.e if I deselect Automotive category then partials A.1 and A.2 are removed.

My question is - how to achieve this thing, where model is totally different compared to form, with Symfony Form Builder? In summary:

• categories exist only on form, invisible partials only in model
• checkbox value must contain whether it is category or partial
• the number of models and checkboxes are different

What I have right now:

• FormEvent preSubmit listener to convert Category checkboxes to Invisibile partials
• ViewTransformer.reverseTransform transforms (after preSubmit) checkbox values (partial IDs) to existing DocumentPartial record or creates new ones
• ViewTransformer.transform transforms existing DocumentPartials to category and partial checkbox values
• checkboxes are rendered separately, not by Form Builder :)

This whole thing has gotten too complex and Im starting to think I am doing it wrong :) What is the magic method here that "will do it as I need"?