Cannot use from_dataset() with join criteria on capitalized field name

Actually, I was wrong, you can use upper case symbols as hash keys in ruby, so MemberId: field('MemberId') should work find. The only problem was that you needed the field() call on the right side of the hash expression.

As an aside, regarding the arrow syntax, that's ruby's original hash syntax and it allows for keys with spaces and other characters. So for example foo: "bar" is equivalent to "foo" => "bar". You can't however do:

foo bar: "foo bar"

But you can do:

"foo bar" => "foo bar"

Just one of the many idiosyncrasies of ruby.