foo_1.rb

class Foo
  
  # Do some stuff...
  
  # We want to update a field from another model with a value from this model
  after_commit :update_bar_field
  
  # Do more stuff...
  
  private
  
  def update_bar_field
    # We want to update Bar#bar_field_2 with the value of Foo#foo_field_2
    # Bar and foo are related by bar_field_1 and foo_field_1
    # (yes, we might use associations instead, but this is an example ;-) )
    Bar.where(bar_field_1: foo_field_1).update(bar_field_2: foo_field_2)
  end
end

foo_2.rb

class Foo
  
  # Do some stuff...
  
  # We want to update a field from another model with a value from this model
  after_commit :update_bar_field
  
  # Do more stuff...
  
  private
  
  def update_bar_field
    # We want to update Bar#bar_field_2 with the value of Foo#foo_field_2
    # Bar and foo are related by bar_field_1 and foo_field_1
    # (yes, we might use associations instead, but this is an example ;-) )
    Bar.by_bar_field(foo_field_1).update(bar_field_2: Foo.maximum(:foo_field_2)
  end
end

foo_and_bar.rb

class Foo
  
  # Do some stuff...
  
  # We want to update a field from another model with a value from this model
  after_commit :update_bar_field
  
  # Do more stuff...
  
  private
  
  def update_bar_field
    # We want to update Bar#bar_field_2 with the value of Foo#foo_field_2
    # Bar and foo are related by bar_field_1 and foo_field_1
    # (yes, we might use associations instead, but this is an example ;-) )
    Bar.by_bar_field(foo_field_1).update(bar_field_2: foo_field_2)
  end
end

class Bar
  # ...  
  scope :by_bar_field, -> (:bar_field) { where(:bar_field, bar_field) }
  # ...
end

foo_creator.rb

class FooCreator
  def self.call(foo)
    new(foo).call
  end

  def initialize(foo)
    @foo = foo
    @bar = Bar.by_bar_field(foo.foo_field_1) # This is a violation of Dependency inversion principle, but this is a topic for a different post ;-)
  end

  def call
    @bar.bar_field_2 = @foo.foo_field_2
    Foo.transaction do
      @foo.save!
      @bar.save!
    end
  end
end

class Foo  
  # Do some stuff...  
end

class Bar
  # ...  
  scope :by_bar_field, -> (:bar_field) { where(:bar_field, bar_field) }
  # ...
end