Github user srowen commented on a diff in the pull request:

    https://github.com/apache/spark/pull/21740#discussion_r202531693
  
    --- Diff: 
mllib/src/main/scala/org/apache/spark/mllib/recommendation/MatrixFactorizationModel.scala
 ---
    @@ -75,10 +75,22 @@ class MatrixFactorizationModel @Since("0.8.0") (
         }
       }
     
    +  /** Check for the invalid user. */
    +  private def validateUser(user: Int): Unit = {
    +    require(userFeatures.lookup(user).nonEmpty, s"userId: $user not found 
in the model")
    --- End diff --
    
    The problem is you now look up the user and product twice. Instead, the 
callers should probably check whether `userFeatures.lookup(user)` is empty 
before calling `.head` and otherwise throw the exception.


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