mongodb/jstests/update_arraymatch4.js

t = db.update_arraymatch4
t.drop()

x = { _id : 1 , arr : ["A1","B1","C1"] }
t.insert( x )
assert.eq( x , t.findOne() , "A1" )

x.arr[0] = "A2"
t.update( { arr : "A1" } , { $set : { "arr.$" : "A2" } } )
assert.eq( x , t.findOne() , "A2" )

t.ensureIndex( { arr : 1 } )
x.arr[0] = "A3"
t.update( { arr : "A2" } , { $set : { "arr.$" : "A3" } } )
assert.eq( x , t.findOne() , "A3" ); // SERVER-1055