Rails Active Storage: how to access file contents before it has been saved

[Nerdture]

I am working on building an API front end for a 3rd Party API.  In addition to collecting the file from my user, I also need to post the file to the 3rd Party Service.

The only way I have been able to get the actual file contents is to download it again after it has been saved.

class Template < ApplicationRecord
    has_one_attached :pv_file
...
    post_body << self.pv_file.blob.download

I want to have a before create filter that throws an error if the 3rd Party API rejects the file (e.g. wrong format, service unavailable, etc), but the only way I have found to access the file content is by downloading it again.

How can we access the actual file content before it has been saved and uploaded?

[Mladen Ilic]

Hi Nerdture,

I'm not a core team member, but I believe that there's currently no documented way of accessing a file before the record has been saved (more precisely, before the after_commit callback). There is, however, `attachment_changes` hash, which keeps track of what changes are made. You should be able to access the attachment with `record.attachment_changes['pv_file']`. Keep in mind that this is undocumented internal api, so it could change in the future.

[Nerdture]

Thanks Mladen Ilic for your reply.  

I think I am getting closer to the solution I need.

You should be able to access the attachment with `record.attachment_changes['pv_file']`. 

I must be doing something wrong, as I can't figure out how to access the actual file contents/octet-stream.

post_body << self.attachment_changes['pv_file']

throws Zlib::Data Error incorrect header check

post_body << self.attachment_changes['pv_file'].blob.download

throws Aws::S3::Errors::NoSuchKey

post_body << self.attachment_changes['pv_file'].blob.open

throws ActiveStorage::FileNotFoundError

I am sure I am showing my Ruby ignorance here.

Any help is appriciated.

[Mladen Ilic]

Hi again Nerdture,

According to the implementation of ActiveStorage::Attached::Changes::CreateOne, you should be able to access instance of ActionDispatch::Http::UploadedFile with following:

self.attachment_changes['pv_file'].attachable

In you examples, you are trying to download file from a storage service to a tempfile before it has been uploaded, so storage service fails to locate it.

Best,

[Nerdture]

Thanks again Mladen Ilic for your help.

For anyone that finds this, in order to build the multipart form-data, I needed to use the following to get the Octet-stream.

post_body << self.attachment_changes['pv_file'].attachable.read

Hopefully we will eventually get a permanent/documented way to access the data pre-save.