cheat sheets.

    * :class_name - specify the class name of the association. Use it only if
    that name can’t be inferred from the association name. So
    has_and_belongs_to_many :projects will by default be linked to the Project
    class, but if the real class name is SuperProject, you’ll have to specify
    it with this option.
    * :join_table - specify the name of the join table if the default based on
    lexical order isn’t what you want. WARNING: If you’re overwriting the
    table name of either class, the table_name method MUST be declared
    underneath any has_and_belongs_to_many declaration in order to work.
    * :foreign_key - specify the foreign key used for the association. By
    default this is guessed to be the name of this class in lower-case and "_id"
    suffixed. So a Person class that makes a has_and_belongs_to_many association
    will use "person_id" as the default foreign_key.
    * :association_foreign_key - specify the association foreign key used for
    the association. By default this is guessed to be the name of the associated
    class in lower-case and "_id" suffixed. So if the associated class is
    Project, the has_and_belongs_to_many association will use "project_id" as
    the default association foreign_key.
    * :conditions - specify the conditions that the associated object must meet
    in order to be included as a "WHERE" sql fragment, such as "authorized = 1".
    * :order - specify the order in which the associated objects are returned as
    a "ORDER BY" sql fragment, such as "last_name, first_name DESC"
    * :uniq - if set to true, duplicate associated objects will be ignored by
    accessors and query methods
    * :finder_sql - overwrite the default generated SQL used to fetch the
    association with a manual one
    * :delete_sql - overwrite the default generated SQL used to remove links
    between the associated classes with a manual one
    * :insert_sql - overwrite the default generated SQL used to add links
    between the associated classes with a manual one
    * :extend - anonymous module for extending the proxy, see "Association
    extensions".
    * :include - specify second-order associations that should be eager loaded
    when the collection is loaded.
    * :group: An attribute name by which the result should be grouped. Uses the
    GROUP BY SQL-clause.
    * :limit: An integer determining the limit on the number of rows that should
    be returned.
    * :offset: An integer determining the offset from where the rows should be
    fetched. So at 5, it would skip the first 4 rows.
    * :select: By default, this is * as in SELECT * FROM, but can be changed if
    you for example want to do a join, but not include the joined columns.