1stSolo

1stSolo

Does lock_version need to be cast() into ChangeSet, or doing

changeset
|> optimistic_lock(:lock_version)

is enough?

I mean, AFAIK every attempt to update an object should have lock_version in the attributes map, so how does Ecto know what value to use for lock_version if we don’t put it into changeset by casting it?..

But when I look at examples online and in documentation all I see is ChangeSet.optimistic_lock/3 being called.

First Post!

woylie

woylie

It just gets the field from the changeset source:

https://github.com/elixir-ecto/ecto/blob/4a143348ab89e0174a1a5082fd47d868ebd9689b/lib/ecto/changeset.ex#L3418

You don’t need to cast just to access the source data. And you don’t want to cast the lock version, because you don’t want the value to be set from the parameters.

Most Liked

1stSolo

1stSolo OP

In my case I have a absinthe client that pulls the object from the backend via graphql, and edits some fields on the object. The save operation takes id of the object, the changed field, e.g. description, and lockVersion from the last time the data was pulled. Multiple users can edit the same object, hence optimistic locking to prevent overwriting someone else’s changes.
So lockVersion actually comes as part of parameters/arguments along with new value for description.
Therefore I thought the right way to do it is to

%Object{id: params.id}
|> Ecto.Changeset.cast(params.data, [:description, :lock_version])
|> Ecto.Changeset.optimistic_lock(:lock_version)

which should result in Ecto preparing a SQL statement such as
UPDATE Object SET description=?, lock_version=? WHERE id=? AND lock_version=?, and binding params.data.description into 1st arg, incremented lock_version into second arg, params.id into 3rd arg and lockVersion from the client side into the last arg. Then if updated record count returned by the database driver is 1 then update succeeded and if it’s 0 then Ecto would raise StaleStateError.

So I actually do want to set lockVersion from the params. Is that incorrect?

Last Post!

woylie

woylie

Yes, in your case, you’d want to cast the lock version indeed.

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