In releases prior to 2.6 the following statement would work

    "read": { "$sum": { "$eq": [ "$read", true ] } }

Where essentially the "$eq" operation evaluated to true or false and was included in the $sum as either 1 or 0.

This is always evaluating to 0 when applied to the $sum in 2.6 and now requires wrapping with a $cond operator to produce the desired result:

    "read": { "$sum": { "$cond": [ "$read", 1, 0 ] } }

So I am wondering if this is by design or a bug. It does not seem to be documented as a changed behavior