Plaintiffs often file lawsuits which eventually contain frivolous as well as non-frivolous claims. When the defendant prevails in a civil rights suit which contains both types of claims, what is the court to do when the defendant requests its attorneys' fees as the prevailing party?

Generally, in certain civil rights cases the "prevailing party" may be entitled to "reasonable attorneys' fees" under 42 U.S.C. 1988. Typically, the plaintiff is in the position of requesting it's fees, but when the defendant prevails courts have held defendants may be entitled to it's fees only when the claims are frivolous, unreasonable or without foundation. Can the defendant in such a mixed case seek all of it's fees from the plaintiff?