Get Join instance for attribute

To work with foo = RelatedJoin('Song') we use .foo, .addSong and .removeSong. But those names are configurable. How do I get them when all I know is "foo"?

One way seems to be sqlmeta.joins[0].performJoin, .add and .remove. Is that an internal API, and subject to change? How do I get the list index?
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