t = db.index_elemmatch1
t.drop()

x = 0
y = 0
for ( a=0; a<100; a++ ){
    for ( b=0; b<100; b++ ){
        t.insert( { a : a , b : b % 10 , arr : [ { x : x++ % 10 , y : y++ % 10 } ] } )
    }
}

t.ensureIndex( { a : 1 , b : 1 } )
t.ensureIndex( { "arr.x" : 1 , a : 1 } )

assert.eq( 100 , t.find( { a : 55 } ).itcount() , "A1" );
assert.eq( 10 , t.find( { a : 55 , b : 7 } ).itcount() , "A2" );

q = { a : 55 , b : { $in : [ 1 , 5 , 8 ] } }
assert.eq( 30 , t.find( q ).itcount() , "A3" )

q.arr = { $elemMatch : { x : 5 , y : 5 } }
assert.eq( 10 , t.find( q ).itcount() , "A4" )

function nscannedForCursor( explain, cursor ) {
    plans = explain.allPlans;
    for( i in plans ) {
        if ( plans[ i ].cursor == cursor ) {
            return plans[ i ].nscanned;
        }
    }
    return -1;
}

// QUERY_MIGRATION:
// Query is {a:55, b:{$in:[1,5,8]}, arr:{$elemMatch:{x:5, y:5}}}.  New system considers indices over
// {a:1, b:1} and {arr.x:1, a:1}.  For the latter case, the enumerator doesn't look down the
// elemMatch tree to seek out compounding.
//
//assert.eq( t.find(q).itcount(),
//         nscannedForCursor( t.find(q).explain(true), 'BtreeCursor arr.x_1_a_1' ), "A5" );

printjson(t.find(q).explain());
print("Num results:");
assert.eq(10, t.find(q).itcount());
printjson(t.find(q).itcount());