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docs: clarify index shifting in cast_assoc/3 when using drop_param #4705

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josevalim merged 3 commits into from
Feb 16, 2026
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docs: clarify index shifting in cast_assoc/3 when using drop_param #4705

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docs: clarify index shifting in cast_assoc/3 when using drop_param
JuanmiAcosta Feb 15, 2026
1268a5a
fix : remove unnecessary sort_param reference from documentation
JuanmiAcosta Feb 15, 2026
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josevalim Feb 16, 2026
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16 changes: 16 additions & 0 deletions lib/ecto/changeset.ex
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Original file line number Diff line number Diff line change
Expand Up @@ -1160,6 +1160,22 @@ defmodule Ecto.Changeset do
changeset
|> cast_assoc(:children, sort_param: ..., with: &child_changeset/3)

When using `:drop_param`, the remaining elements are re-indexed sequentially
starting from 0. For example, if you have the following parameters:

%{
"addresses" => %{
0 => %{"street" => "First St"},
1 => %{"street" => "Second St"},
2 => %{"street" => "Third St"}
},
"addresses_drop" => [1]
}

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.

These parameters can be powerful in certain UIs as it allows you to decouple
the sorting and replacement of the data from its representation.

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