Add support for before build callback

lib/factory_bot/callback.rb

@@ -7,6 +7,10 @@ def initialize(name, block)
       @block = block
     end
 
+    def run_before_build
+      syntax_runner.instance_exec(&block)
+    end
+
     def run(instance, evaluator)
       case block.arity
       when 1, -1, -2 then syntax_runner.instance_exec(instance, &block)
@@ -20,6 +24,10 @@ def ==(other)
         block == other.block
     end
 
+    def before_build?
+      name == :before_build
+    end
+
     protected
 
     attr_reader :block

lib/factory_bot/factory.rb

@@ -32,6 +32,7 @@ def build_class
     def run(build_strategy, overrides, &block)
       block ||= ->(result) { result }
       compile
+      run_before_build_callbacks
 
       strategy = StrategyCalculator.new(build_strategy).strategy.new
 
@@ -139,6 +140,12 @@ def compiled_constructor
       hierarchy_instance.constructor
     end
 
+    def run_before_build_callbacks
+      callbacks.each do |callback|
+        callback.run_before_build if callback.before_build?
+      end
+    end
+
     private
 
     def assert_valid_options(options)

spec/acceptance/callbacks_spec.rb

@@ -249,3 +249,33 @@ def name
     expect(build(:company).name).to eq "ACME SUPPLIERS"
   end
 end
+
+describe "before build callback" do
+  class TitleSetter
+    def self.title=(new_title)
+      @@title = new_title
+    end
+
+    def self.title
+      @@title
+    end
+  end
+
+  before do
+    define_model("Article", title: :string)
+
+    FactoryBot.define do
+      factory :article_with_before_callbacks, class: :article do
+        before(:build) { TitleSetter.title = "title from before build" }
+        after(:build) { TitleSetter.title = "title from after build" }
+
+        title { TitleSetter.title }
+      end
+    end
+  end
+
+  it "runs the before callback" do
+    article = FactoryBot.build(:article_with_before_callbacks)
+    expect(article.title).to eq("title from before build")
+  end
+end

spec/factory_bot/callback_spec.rb

@@ -24,4 +24,15 @@
     FactoryBot::Callback.new(:after_create, ->(one, two) { ran_with = [one, two] }).run(:one, :two)
     expect(ran_with).to eq [:one, :two]
   end
+
+  it "runs run_before_build callback without attributes" do
+    ran_with = nil
+    FactoryBot::Callback.new(:before_build, -> { ran_with = "before build" }).run_before_build
+    expect(ran_with).to eq "before build"
+  end
+
+  it "#before_build?" do
+    expect(FactoryBot::Callback.new(:before_build, -> {}).before_build?).to be true
+    expect(FactoryBot::Callback.new(:after_create, -> {}).before_build?).to be false
+  end
 end