Struct representing a HasOne (or belongs to) relationship.

The fields are:

• serializer - A Serializer (often a PhoenixView) implementing the JaSerializer.Serializer behaviour.
• include - Should this relationship be included (sideloaded) by default. Overriden by include opt to JaSerializer.format/4
• data - A struct representing the data for serialization.
• identifiers - Should "resource identifiers be included, options are :when_included and :always. Defaults to :always
• links - A keyword list of links, self and related are most common.
• name - Name of the relationship, automatically set.

Used when defining relationships without the DSL using the JaSerializer.relationships/2 callback. For example:

def relationships(article, _conn) do
  %{
    comments: %HasOne{
      serializer:  MyApp.CommentView,
      include:     true,
      data:        article.comments,
    }
  }
end

See JaSerializer.DSL.has_many/2 for information on defining different types of relationships.