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For a project we’ve worked on recently we were using attachment_fu to handle file uploads in a series of models in part because of how easy it makes automatic thumbnail generation. As per normal practices we had certain fields, e.g. a “title” field, set to be both required and unique, but we started getting all these errors related to the title field being missing any time we’d create a new record and upload a file.

After much fiddling around with the code it finally dawned upon us - attachment_fu’s amazing abilities create partially empty records to store thumbnails, which will obviously trip up the validators. You see, when attachment_fu creates the requested thumbnails it creates additional records in the same model and uses the parent_id to link the thumbnail(s) to the master record (the one corresponding to the original upload); however it doesn’t automatically fill in the other fields as that would be too complicated to automate, so any fields other than those attachment_fu itself uses will be blank.

The way around this unanticipated feature is to restrict the validations to only records that have a blank parent_id record, i.e. only the master records, e.g.:

validates_presence_of :title, :unless => :parent_id?
validates_uniqueness_of :title, :unless => :parent_id?

Also, bearing in mind how attachment_fu handles automatically generated thumbnails it is strongly advised to keep models with attachments highly normalized (to not have many additional fields in these models not directly used by attachment_fu), otherwise you’ll end up with a large amount of waste, which can only slow down queries.

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