docs: clarify index shifting in cast_assoc/3 when using drop_param#4705
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josevalim merged 3 commits intoelixir-ecto:masterfrom Feb 16, 2026
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| The element originally at index `2` ("Third St") will be passed to the `:with` | ||
| function with position `1`, because the list is compacted after the drop | ||
| operation. If you need to preserve the original semantic positions, ensure | ||
| you also provide a `:sort_param`. |
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Can we preserve the original positions even with sort_param? If an element is removed, no element can ever be at index 2, right?
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You're right, the positions are not preserved since the collection is compacted into a list. I'm removing that last line. Thanks for the feedback, I appreciate it! -> 1268a5a
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Thank you and congratulations @JuanmiAcosta on your first PR! ❤️ I have dropped one tiny question! |
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While the behavior of
:drop_paramand:sort_paramis documented, this PR adds a specific clarification regarding the index shifting that occurs when elements are dropped but not explicitly re-sorted.Context:
When using a map for many-style associations, Ecto compacts the remaining elements into a sequential list after applying
:drop_param. As a result, the index passed to an arity-3:withfunction reflects the final position in the list, which might differ from the original map key.Contribution:
Explicitly mentions that remaining elements are re-indexed sequentially starting from 0.
Adds a visual example to illustrate the difference between the original parameter key and the final position received by the changeset function.
This addition aims to help developers avoid subtle bugs when using the :with option to persist order (e.g., in a :position column), making the underlying list transformation more explicit.
Note: This is my first contribution to Ecto (and to an open-source project). I've found this specific case while working on a production system and thought it would be a valuable addition to the documentation for other developers. Feedback is more than welcome!