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A foundation for building Elixir Phoenix apps — SaaS, social networks, ERP systems, marketplaces, and more
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Files
lib/phoenix_kit/utils/reorder.ex
defmodule PhoenixKit.Utils.Reorder do
@moduledoc """
Two-phase index rewrite for drag-to-reorder list views.
Given a list of UUIDs in their new display order and an Ecto schema,
rewrites a position field on the matching rows to `1..N` matching
the order of the input list. The write runs in two passes inside a
transaction — first to negative indices, then to positive — so a
unique index on the position column (should one ever be added)
wouldn't trip mid-update.
Consumers that need pre-write validation (scope checks, permission
guards) or post-write side effects (activity logging, PubSub
broadcasts) should wrap this helper rather than fold the logic in
here. `PhoenixKitProjects.reorder_projects/2` is the reference
consumer doing exactly that — this module owns only the index-rewrite
primitive.
Non-UUID entries in the payload are silently filtered (a stale or
malformed drop event can't poison the rewrite). Duplicates dedup
last-write-wins via `Enum.uniq/1`.
## Example
defmodule MyApp.Endpoints do
alias PhoenixKit.Utils.Reorder
def reorder_endpoints(ordered_ids) do
Reorder.reorder(MyApp.Endpoint, ordered_ids, :sort_order, repo: repo())
end
end
"""
import Ecto.Query
alias PhoenixKit.Utils.UUID
@default_max_uuids 500
@type result :: {:ok, non_neg_integer()} | {:error, :too_many_uuids}
@doc """
Rewrites `field` on the rows whose `uuid` appears in `ordered_ids`,
setting it to each UUID's 1-based position in the list.
Returns `{:ok, count}` where `count` is the number of rows actually
updated in the positive-write phase (matches `Repo.update_all`'s
count semantics — UUIDs in the payload that don't resolve to real
rows aren't counted). Returns `{:error, :too_many_uuids}` when the
dedup'd payload exceeds the configured cap. An empty / fully-filtered
payload returns `{:ok, 0}`.
## Options
- `:repo` — the Ecto repo to use. Defaults to
`PhoenixKit.RepoHelper.repo/0` so it picks up the host app's repo.
- `:max_uuids` — payload cap, checked after dedup. Default `500`.
Guards against runaway drop events from a misbehaving client.
"""
@spec reorder(module(), [String.t()], atom(), keyword()) :: result()
def reorder(schema, ordered_ids, field, opts \\ [])
when is_atom(schema) and is_list(ordered_ids) and is_atom(field) do
max = Keyword.get(opts, :max_uuids, @default_max_uuids)
repo = Keyword.get(opts, :repo, PhoenixKit.RepoHelper.repo())
case dedupe_uuids(ordered_ids) do
[] ->
{:ok, 0}
uuids when length(uuids) > max ->
{:error, :too_many_uuids}
uuids ->
{:ok, count} =
repo.transaction(fn ->
pairs = Enum.with_index(uuids, 1)
_ = write_phase(repo, schema, pairs, field, -1)
write_phase(repo, schema, pairs, field, 1)
end)
{:ok, count}
end
end
defp write_phase(repo, schema, pairs, field, sign) do
Enum.reduce(pairs, 0, fn {uuid, idx}, total ->
{n, _} =
from(r in schema, where: r.uuid == ^uuid)
|> repo.update_all(set: [{field, sign * idx}])
total + n
end)
end
defp dedupe_uuids(ids) do
ids
|> Enum.filter(&UUID.valid?/1)
|> Enum.uniq()
end
end