# Objective
The methods and commands `replace_related` and
`replace_related_with_difference` may cause data stored at the
`RelationshipTarget` be lost when all original children are removed
before new children are added.
Part of https://github.com/bevyengine/bevy/issues/19589
## Solution
Fix the issue, either by removing the old children _after_ adding the
new ones and not _before_ (`replace_related_with_difference`) or by
taking the whole `RelationshipTarget` to modify it, not only the inner
collection (`replace_related`).
## Testing
I added a new test asserting the data is kept. I also added a general
test of these methods as they had none previously.
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Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
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