[Bug] Child sorting doesn't take immutable entities into account

[lutter]

Bug report

When sorting by a child field, this line needs to take into account whether the child table is mutable or immutable.

Some information to help us out

• [ ] Tick this box if this bug is caused by a regression found in the latest release.
• [ ] Tick this box if this bug is specific to the hosted service.
• [X] I have searched the issue tracker to make sure this issue is not a duplicate.