Ruby on Rails. How do I use the Active Record .build method in a :belongs to relationship?

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Where it is documented:

From the API documentation under the has_many association in “Module ActiveRecord::Associations::ClassMethods“

collection.build(attributes = {}, …)
Returns one or more new objects of the
collection type that have been
instantiated with attributes and
linked to this object through a
foreign key, but have not yet been
saved. Note: This only works if an
associated object already exists, not
if it‘s nil!

The answer to building in the opposite direction is a slightly altered syntax. In your example with the dogs,

Class Dog
   has_many :tags
   belongs_to :person
end

Class Person
  has_many :dogs
end

d = Dog.new
d.build_person(:attributes => "go", :here => "like normal")

or even 

t = Tag.new
t.build_dog(:name => "Rover", :breed => "Maltese")

You can also use create_dog to have it saved instantly (much like the corresponding “create” method you can call on the collection)

How is rails smart enough? It’s magic (or more accurately, I just don’t know, would love to find out!)

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