Collection in a command

Languages Application language
,
1

I noticed it’s not possible to use a command to add data to a collection with a sub-collection straight away. Instead I should use a second command to add data to the sub-collection.
Why is this not possible? Or what am I doing wrong?
Example (warning: with pseudo-code!):

'Orders': collection ['Order'] {
   'Order': text
   'Order line': collection ['Line'] {
      'Line': text
   }
}

'Add order': command {
   'Order': text
   'Order line': collection ['Line'] {
      'Line': text
   }
} => update .'Orders' = create (
   'Order' = @ .'Order'
   'Order line' = @ .'Order line' as $ ( //pseudo-code!
      'Line' = $ .'Line'                 //pseudo-code!
   )                                     //pseudo-code!
)
2

You cannot assign a collection property to another (differently typed) collection property. Instead you have to walk over the entries of a source collection, and use create to create entries in a target collection, like this:

   'Order line' = walk @ .'Order line' as $ ( create (
      'Line' = $ .'Line'                 
   ) )