feature Add file_fixture helper to syntax methods

Why:
This is to add simple support to files on factories. This is the
original [issue](#1282).

What:
It is a common behavior to interact with files when creating factories,
and it is used heavily, as demonstrated with the many examples in the
issue, this is a first draft and I would love to see more use-cases to
add to the acceptance test. For the time being we have at least the base
case added.

lib/factory_bot.rb

@@ -46,6 +46,7 @@
 require "factory_bot/linter"
 require "factory_bot/version"
 require "factory_bot/internal"
+require "factory_bot/file_loader"
 
 module FactoryBot
   Deprecation = ActiveSupport::Deprecation.new("6.0", "factory_bot")

lib/factory_bot/errors.rb

@@ -22,4 +22,6 @@ class MethodDefinitionError < RuntimeError; end
 
   # Raised when any factory is considered invalid
   class InvalidFactoryError < RuntimeError; end
+
+  class FileDoesNotExistError < RuntimeError; end
 end

lib/factory_bot/file_loader.rb

@@ -0,0 +1,39 @@
+require "tempfile"
+
+module FactoryBot
+  class FileLoader
+    attr_reader :tempfile, :original_filename
+
+    def initialize(file_path)
+      from_file_path(file_path)
+    end
+
+    def path
+      tempfile.path
+    end
+
+    def method_missing(method_name, *args, &block) # rubocop:disable Style/MethodMissing
+      tempfile.public_send(method_name, *args, &block)
+    end
+
+    def respond_to_missing?(method_name, include_private = false)
+      tempfile.respond_to?(method_name, include_private) || super
+    end
+
+    private
+
+    def from_file_path(path)
+      raise FileDoesNotExistError unless ::File.exist?(path)
+
+      @original_filename = ::File.basename(path)
+      extension = ::File.extname(@original_filename)
+
+      @tempfile = Tempfile.new(
+        [::File.basename(@original_filename, extension), extension],
+      )
+      @tempfile.set_encoding(Encoding::BINARY) if @tempfile.respond_to?(:set_encoding)
+
+      FileUtils.copy_file(path, @tempfile.path)
+    end
+  end
+end

lib/factory_bot/syntax/methods.rb

@@ -129,6 +129,20 @@ def generate_list(name, count)
           Internal.sequence_by_name(name).next
         end
       end
+
+      # Creates a copy of the file passed to attach to the attribute
+      #
+      # Arguments:
+      #   filepath: (String)
+      #     The path for the file that will be loaded. Should be a valid path
+      #     for a file that exists at that location.
+      #
+      # Returns:
+      #   An instance of the file loader class pointing to the file at the
+      #   provided path.
+      def file_fixture(file_path)
+        FileLoader.new(file_path)
+      end
     end
   end
 end

spec/acceptance/file_attributes_spec.rb

@@ -0,0 +1,20 @@
+describe "file attributes" do
+  context "when an attribute uses a file" do
+    it "assigns an file to the attribute" do
+      define_class("Post") do |_class|
+        attr_accessor :attachment
+      end
+      filename = File.expand_path("spec/support/text_file.txt")
+      file_contents = File.read(filename)
+
+      FactoryBot.define do
+        factory :post do
+          attachment { file_fixture(filename) }
+        end
+      end
+      post = FactoryBot.build(:post)
+      expect(post.attachment.original_filename).to eq ::File.basename filename
+      expect(File.read(post.attachment)).to eq file_contents
+    end
+  end
+end

spec/factory_bot/file_loader_spec.rb

@@ -0,0 +1,21 @@
+describe FactoryBot::FileLoader do
+  it "creates a tempfile copy from the original" do
+    original_file_path = File.expand_path("spec/support/text_file.txt")
+    original_file_contents = File.read(original_file_path)
+    file_loader = FactoryBot::FileLoader.new(original_file_path)
+
+    expect(original_file_contents).to eq File.read(file_loader.tempfile)
+  end
+
+  it "delegates all the methods to the tempfile" do
+    original_file_path = File.expand_path("spec/support/text_file.txt")
+    file_loader = FactoryBot::FileLoader.new(original_file_path)
+
+    expect(file_loader.path).to eq file_loader.tempfile.path
+  end
+
+  it "raises an exception is the file path does not exists" do
+    expect { FactoryBot::FileLoader.new("nonexistent.path") }.
+      to raise_error FactoryBot::FileDoesNotExistError
+  end
+end

spec/support/text_file.txt

@@ -0,0 +1 @@
+Test Content