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Image blobs can have variants that are the result of a set of transformations applied to the original. These variants are used to create thumbnails, fixed-size avatars, or any other derivative image from the original.

Variants rely on MiniMagick for the actual transformations of the file, so you must add gem "mini_magick" to your Gemfile if you wish to use variants.

Note that to create a variant it's necessary to download the entire blob file from the service and load it into memory. The larger the image, the more memory is used. Because of this process, you also want to be considerate about when the variant is actually processed. You shouldn't be processing variants inline in a template, for example. Delay the processing to an on-demand controller, like the one provided in ActiveStorage::VariantsController.

To refer to such a delayed on-demand variant, simply link to the variant through the resolved route provided by Active Storage like so:

<%= image_tag url_for(Current.user.avatar.variant(resize: "100x100")) %>

This will create a URL for that specific blob with that specific variant, which the ActiveStorage::VariantsController can then produce on-demand.

When you do want to actually produce the variant needed, call processed. This will check that the variant has already been processed and uploaded to the service, and, if so, just return that. Otherwise it will perform the transformations, upload the variant to the service, and return itself again. Example:

avatar.variant(resize: "100x100").processed.service_url

This will create and process a variant of the avatar blob that's constrained to a height and width of 100. Then it'll upload said variant to the service according to a derivative key of the blob and the transformations.

A list of all possible transformations is available at www.imagemagick.org/script/mogrify.php. You can combine as many as you like freely:

avatar.variant(resize: "100x100", monochrome: true, flip: "-90")
Methods
I
K
N
P
S
Attributes
[R] blob
[R] variation
Class Public methods
new(blob, variation_or_variation_key)
# File activestorage/app/models/active_storage/variant.rb, line 40
def initialize(blob, variation_or_variation_key)
  @blob, @variation = blob, ActiveStorage::Variation.wrap(variation_or_variation_key)
end
Instance Public methods
image()

Returns the receiving variant. Allows ActiveStorage::Variant and ActiveStorage::Preview instances to be used interchangeably.

# File activestorage/app/models/active_storage/variant.rb, line 68
def image
  self
end
key()

Returns a combination key of the blob and the variation that together identifies a specific variant.

# File activestorage/app/models/active_storage/variant.rb, line 51
def key
  "variants/#{blob.key}/#{Digest::SHA256.hexdigest(variation.key)}"
end
processed()

Returns the variant instance itself after it's been processed or an existing processing has been found on the service.

# File activestorage/app/models/active_storage/variant.rb, line 45
def processed
  process unless processed?
  self
end
service_url(expires_in: service.url_expires_in, disposition: :inline)

Returns the URL of the variant on the service. This URL is intended to be short-lived for security and not used directly with users. Instead, the service_url should only be exposed as a redirect from a stable, possibly authenticated URL. Hiding the service_url behind a redirect also gives you the power to change services without updating all URLs. And it allows permanent URLs that redirect to the service_url to be cached in the view.

Use url_for(variant) (or the implied form, like +link_to variant+ or +redirect_to variant+) to get the stable URL for a variant that points to the ActiveStorage::VariantsController, which in turn will use this service_call method for its redirection.

# File activestorage/app/models/active_storage/variant.rb, line 63
def service_url(expires_in: service.url_expires_in, disposition: :inline)
  service.url key, expires_in: expires_in, disposition: disposition, filename: blob.filename, content_type: blob.content_type
end