Module: FileBlobs::ActiveRecordExtensions::ClassMethods

Defined in:
lib/file_blobs_rails/active_record_extensions.rb

Overview

module FileBlobs::ActiveRecordExtensions

Instance Method Summary collapse

Instance Method Details

#has_file_blob(attribute_name = 'file', options = {}) ⇒ Object

Creates a reference to a FileBlob storing a file.

‘has_file_blob :file` creates the following

  • file - synthetic accessor

  • file_blob - belongs_to relationship pointing to a FileBlob

  • file_blob_id - attribute used by the belongs_to relationship; stores the SHA-256 of the file’s contents

  • file_size - attribute storing the file’s length in bytes; this is stored in the model as an optimization, so the length can be displayed / used for decisions without fetching the blob model storing the contents

  • file_mime_type - attribute storing the MIME type associated with the file; this is stored outside the blob model because it is possible to have the same bytes uploaded with different MIME types

  • file_original_name - attribute storing the name supplied by the browser that uploaded the file; this should not be trusted, as it is controlled by the user

Parameters:

  • attribute_name (String) (defaults to: 'file') —

    the name of the relationship pointing to the file blob, and the root of the names of the related attributes

  • options (Hash{Symbol, Object}) (defaults to: {})

Options Hash (options):

  • blob_model (String) —

    the name of the model used to store the file’s contents; defaults to ‘FileBlob’

  • allow_nil (Boolean) —

    if true, allows saving a model without an associated file



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# File 'lib/file_blobs_rails/active_record_extensions.rb', line 37

def has_file_blob(attribute_name = 'file', options = {})
  blob_model = (options[:blob_model] || 'FileBlob'.freeze).to_s
  allow_nil = options[:allow_nil] || false

  self.class_eval <<ENDRUBY, __FILE__, __LINE__ + 1
    # Saves the old blob model id, so the de-referenced blob can be GCed.
    before_save :#{attribute_name}_stash_old_blob, on: :update

    # Checks if the de-referenced FileBlob in an update should be GCed.
    after_update :#{attribute_name}_maybe_garbage_collect_old_blob

    # Checks if the FileBlob of a deleted entry should be GCed.
    after_destroy :#{attribute_name}_maybe_garbage_collect_blob

    # The FileBlob storing the file's content.
    belongs_to :#{attribute_name}_blob,
        { class_name: #{blob_model.inspect} }, -> { select :id }
    validates_associated :file_blob

    class #{attribute_name.to_s.classify}Proxy < FileBlobs::FileBlobProxy
      # Creates a proxy for the given model.
      #
      # The proxied model remains constant for the life of the proxy.
      def initialize(owner)
        @owner = owner
      end

      # Virtual attribute that proxies to the model's _blob attribute.
      #
      # This attribute does not have a corresponding setter because a _blob
      # setter would encourage sub-optimal code. The owner model's file blob
      # setter should be used instead, as it has a fast path that avoids
      # fetching the blob's data. By comparison, a _blob setter would always
      # have to fetch the blob data, to determine the blob's size.
      def blob
        @owner.#{attribute_name}_blob
      end

      # Virtual attribute that proxies to the model's _mime_type attribute.
      def mime_type
        @owner.#{attribute_name}_mime_type
      end
      def mime_type=(new_mime_type)
        @owner.#{attribute_name}_mime_type = new_mime_type
      end

      # Virtual attribute that proxies to the model's _original_name attribute.
      def original_name
        @owner.#{attribute_name}_original_name
      end
      def original_name=(new_name)
        @owner.#{attribute_name}_original_name = new_name
      end

      # Virtual getter that proxies to the model's _size attribute.
      #
      # This attribute does not have a corresponding setter because the _size
      # attribute automatically tracks the _data attribute, so it should not
      # be set on its own.
      def size
        @owner.#{attribute_name}_size
      end

      # Virtual attribute that proxies to the model's _blob_id attribute.
      #
      # This attribute is an optimization that allows some code paths to
      # avoid fetching the associated blob model. It should only be used in
      # these cases.
      #
      # This attribute does not have a corresponding setter because the
      # contents blob should be set using the model's _blob attribute (with
      # the blob proxy), which updates the model _size attribute and checks
      # that the blob is an instance of the correct blob model.
      def blob_id
        @owner.#{attribute_name}_blob_id
      end

      # Virtual attribute that proxies to the model's _data attribute.
      def data
        @owner.#{attribute_name}_data
      end
      def data=(new_data)
        @owner.#{attribute_name}_data = new_data
      end

      # Reflection.
      def blob_class
        #{blob_model}
      end
      def allow_nil?
        #{allow_nil}
      end
      attr_reader :owner
    end

    # Getter for the file's convenience proxy.
    def #{attribute_name}
      @_#{attribute_name}_proxy ||=
          #{attribute_name.to_s.classify}Proxy.new self
    end

    # Convenience setter for all the file attributes.
    #
    # @param {ActionDispatch::Http::UploadedFile, Proxy} new_file either an
    #     object representing a file uploaded to a controller, or an object
    #     obtained from another model's blob accessor
    def #{attribute_name}=(new_file)
      if new_file.respond_to? :read
        # ActionDispatch::Http::UploadedFile
        self.#{attribute_name}_mime_type = new_file.content_type
        self.#{attribute_name}_original_name = new_file.original_filename
        self.#{attribute_name}_data = new_file.read
      elsif new_file.respond_to? :blob_class
        # Blob owner proxy.
        self.#{attribute_name}_mime_type = new_file.mime_type
        self.#{attribute_name}_original_name = new_file.original_name
        if new_file.blob_class == #{blob_model}
          # Fast path: when the two files are backed by the same blob table,
          # we can create a new reference to the existing blob.
          self.#{attribute_name}_blob_id = new_file.blob_id
          self.#{attribute_name}_size = new_file.size
        else
          # Slow path: we need to copy data across blob tables.
          self.#{attribute_name}_data = new_file.data
        end
      elsif new_file.nil?
        # Reset everything to nil.
        self.#{attribute_name}_mime_type = nil
        self.#{attribute_name}_original_name = nil
        self.#{attribute_name}_data = nil
      else
        raise ArgumentError, "Invalid file_blob value: \#{new_file.inspect}"
      end
    end

    # Convenience getter for the file's content.
    #
    # @return [String] a string with the binary encoding that holds the
    #     file's contents
    def #{attribute_name}_data
      # NOTE: we're not using the ActiveRecord association on purpose, so
      #       that the large FileBlob doesn't hang off of the object
      #       referencing it; this way, the blob's data can be
      #       garbage-collected by the Ruby VM as early as possible
      if blob_id = #{attribute_name}_blob_id
        blob = #{blob_model}.where(id: blob_id).first!
        blob.data
      else
        nil
      end
    end

    # Convenience setter for the file's content.
    #
    # @param new_blob_contents [String] a string with the binary encoding
    #     that holds the new file contents to be stored by this model
    def #{attribute_name}_data=(new_blob_contents)
      sha = new_blob_contents && #{blob_model}.id_for(new_blob_contents)
      return if self.#{attribute_name}_blob_id == sha

      if sha && #{blob_model}.where(id: sha).length == 0
        self.#{attribute_name}_blob = #{blob_model}.new id: sha,
            data: new_blob_contents
      else
        self.#{attribute_name}_blob_id = sha
      end

      self.#{attribute_name}_size =
          new_blob_contents && new_blob_contents.bytesize
    end

    # Saves the old blob model id, so the de-referenced blob can be GCed.
    def #{attribute_name}_stash_old_blob
      @_#{attribute_name}_old_blob_id = #{attribute_name}_blob_id_change &&
          #{attribute_name}_blob_id_change.first
    end
    private :#{attribute_name}_stash_old_blob

    # Checks if the de-referenced blob model in an update should be GCed.
    def #{attribute_name}_maybe_garbage_collect_old_blob
      return unless @_#{attribute_name}_old_blob_id
      old_blob = #{blob_model}.find @_#{attribute_name}_old_blob_id
      old_blob.maybe_garbage_collect
      @_#{attribute_name}_old_blob_id = nil
    end
    private :#{attribute_name}_maybe_garbage_collect_old_blob

    # Checks if the FileBlob of a deleted entry should be GCed.
    def #{attribute_name}_maybe_garbage_collect_blob
      #{attribute_name}_blob && #{attribute_name}_blob.maybe_garbage_collect
    end
    private :#{attribute_name}_maybe_garbage_collect_blob

    unless self.respond_to? :file_blob_id_attributes
      @@file_blob_id_attributes = {}
      cattr_reader :file_blob_id_attributes, instance_reader: false
    end

    unless self.respond_to? :file_blob_eligible_for_garbage_collection?
      # Checks if a contents blob is referenced by a model of this class.
      #
      # @param {FileBlobs::BlobModel} file_blob a blob to be checked
      def self.file_blob_eligible_for_garbage_collection?(file_blob)
        attributes = file_blob_id_attributes[file_blob.class.name]
        file_blob_id = file_blob.id

        # TODO(pwnall): Use or to issue a single SQL query for multiple
        #     attributes.
        !attributes.any? do |attribute|
          where(attribute => file_blob_id).exists?
        end
      end
    end
ENDRUBY

  file_blob_id_attributes[blob_model] ||= []
  file_blob_id_attributes[blob_model] << :"#{attribute_name}_blob_id"

  if !allow_nil
    self.class_eval <<ENDRUBY, __FILE__, __LINE__ + 1
      validates :#{attribute_name}_blob, presence: true
      validates :#{attribute_name}_mime_type, presence: true
      validates :#{attribute_name}_size, presence: true
ENDRUBY
  end
end