Domain model and grouped data
I'm starting to catch the domain model idea but there is one thing that I fail to see how this is realized in domain model - grouped data. The most obvious example is various reports, with or without dynamic column grouping and user specified order of columns.

I'll try to provide specific example. Imagine following domain model: 

    Orders [1-to-many] Positions [1-to-many] Production items

Displayed as simple 3 level list.

    Order 1
    - Position 1
      - Production item 1, length: 2.5, weight: 1.234
      - Production item 2, length: 2.5, weight: 2.345
      - Production item 3, length: 5, weight: 3.456

Then goes the requirement to modify Production level by adding "length" field (just for the display, the Production entity is NOT altered) from Production item and group by this field.

    Order 1
    - Position 1, length: 2.5
      - Production item 1, length: 2.5, weight: 1.234
      - Production item 2, length: 2.5, weight: 2.345
    - Position 1, length: 5
      - Production item 3, length: 5, weight: 3.456

And next requirement is to add another SUMmed field "weight" from Production item level in Production level. Again, only on display.

    Order 1
    - Position 1, length: 2.5, weight: 3.579
      - Production item 1, length: 2.5, weight: 1.234
      - Production item 2, length: 2.5, weight: 2.345
    - Position 1, length: 5, weight: 3.456
      - Production item 3, length: 5, weight: 3.456

What I need to change to achieve this "artifical" model?
Domain model and grouped data
===

I'm starting to catch the domain model idea but there is one thing that I fail to see how this is realized in domain model - grouped data. The most obvious example is various reports, with or without dynamic column grouping and user specified order of columns.

I'll try to provide specific example. Imagine following domain model: 

    Orders [1-to-many] Positions [1-to-many] Production items

Displayed as simple 3 level list.

    Order 1
    - Position 1
      - Production item 1, length: 2.5, weight: 1.234
      - Production item 2, length: 2.5, weight: 2.345
      - Production item 3, length: 5, weight: 3.456

Then goes the requirement to modify Production level by adding "length" field (just for the display, the Production entity is NOT altered) from Production item and group by this field.

    Order 1
    - Position 1, length: 2.5
      - Production item 1, length: 2.5, weight: 1.234
      - Production item 2, length: 2.5, weight: 2.345
    - Position 1, length: 5
      - Production item 3, length: 5, weight: 3.456

And next requirement is to add another SUMmed field "weight" from Production item level in Production level. Again, only on display.

    Order 1
    - Position 1, length: 2.5, weight: 3.579
      - Production item 1, length: 2.5, weight: 1.234
      - Production item 2, length: 2.5, weight: 2.345
    - Position 1, length: 5, weight: 3.456
      - Production item 3, length: 5, weight: 3.456

What I need to change to achieve this "artifical" model?