mongo/jstests/index3.js

t = db.index3;
t.drop();

assert( t.getIndexes().length == 0 );

t.ensureIndex( { name : 1 } );

t.save( { name : "a" } );

t.ensureIndex( { name : 1 } );

assert( t.getIndexes().length == 2 );

assert(t.validate().valid);