Hi,

I noticed that the Medley Relationship Type (http://musicbrainz.org/doc/Medley_Relationship_Type) is lacking some attributes which are available in Performance Relationship Type (http://musicbrainz.org/doc/Performance_Relationship_Type).

Example:
http://musicbrainz.org/release/289ecc02-dce8-44b3-a99e-0d2312d8885b
In this release there is a medley which is played live but I can't set that attribute.

But I think you could find a use-case for every attribute of the Performance Relationship Type which could be used in medleys too. So my suggestion would be to include all attributes, namely:
start date:
Already exists

end date:
Already exists

live:
This is needed for medleys which are played live.

cover:
This might be tricky. If the complete medley consists of covers or non-covers it's no problem. But what if a medley consists of both cover and non-cover songs. I don't have a good idea how to cover (Ba Dum Tss!) that use-case.

partial:
Same goes for this one. While most medleys might just use parts of different songs there might be a good chance that there's medleys which contain complete songs too... Well, and a combination of both.

instrumental:
This has the same problems like the two before.

Long story short: I think at least the "live"-attribute is needed for medleys too.