text: 
  t = db.idhack
  t.drop()
  
  
  t.insert( { _id : { x : 1 } , z : 1 } )
  t.insert( { _id : { x : 2 } , z : 2 } )
  t.insert( { _id : { x : 3 } , z : 3 } )
  t.insert( { _id : 1 , z : 4 } )
  t.insert( { _id : 2 , z : 5 } )
  t.insert( { _id : 3 , z : 6 } )
  
  assert.eq( 2 , t.findOne( { _id : { x : 2 } } ).z , "A1" )
  assert.eq( 2 , t.find( { _id : { $gte : 2 } } ).count() , "A2" )
  assert.eq( 2 , t.find( { _id : { $gte : 2 } } ).itcount() , "A3" )
  
  t.update( { _id : { x : 2 } } , { $set : { z : 7 } } )
  assert.eq( 7 , t.findOne( { _id : { x : 2 } } ).z , "B1" )
  
  t.update( { _id : { $gte : 2 } } , { $set : { z : 8 } } , false , true )
  assert.eq( 4 , t.findOne( { _id : 1 } ).z , "C1" )
  assert.eq( 8 , t.findOne( { _id : 2 } ).z , "C2" )
  assert.eq( 8 , t.findOne( { _id : 3 } ).z , "C3" )
  
  // explain output should show that the ID hack was applied.
  var query = { _id : { x : 2 } };
  var explain = t.find( query ).explain( true );
  print( "explain for " + tojson( query , "" , true ) + " = " + tojson( explain ) );
  assert.eq( 1 , explain.n , "D1" );
  assert.eq( 1 , explain.nscanned , "D2" );
  assert.neq( undefined , explain.cursor , "D3" );
  assert.neq( "" , explain.cursor , "D4" );
  assert.neq( undefined , explain.indexBounds , "D5" );
  assert.neq( {} , explain.indexBounds , "D6" );
  
  // ID hack cannot be used with hint().
  var query = { _id : { x : 2 } };
  var explain = t.find( query ).explain();
  t.ensureIndex( { _id : 1 , a : 1 } );
  var hintExplain = t.find( query ).hint( { _id : 1 , a : 1 } ).explain();
  print( "explain for hinted query = " + tojson( hintExplain ) );
  assert.neq( explain.cursor, hintExplain.cursor, "E1" );
  
  // ID hack cannot be used with skip().
  var skipExplain = t.find( query ).skip(1).explain();
  print( "explain for skip query = " + tojson( skipExplain ) );
  assert.neq( explain.cursor, skipExplain.cursor, "F1" );
  
  // Covered query returning _id field only can be handled by ID hack.
  var coveredExplain = t.find( query, { _id : 1 } ).explain();
  print( "explain for covered query = " + tojson( coveredExplain ) );
  assert.eq( explain.cursor, coveredExplain.cursor, "G1" );
  // Check doc from covered ID hack query.
  assert.eq( { _id : { x: 2 } }, t.findOne( query, { _id : 1 } ), "G2" );
  
  //
  // Non-covered projection for idhack.
  //
  
  t.drop();
  t.insert( { _id: 0, a: 0, b: [ { c: 1 }, { c: 2 } ] });
  t.insert( { _id: 1, a: 1, b: [ { c: 3 }, { c: 4 } ] });
  
  // Simple inclusion.
  assert.eq( { _id: 1, a: 1 }, t.find( { _id: 1 }, { a: 1 } ).next() );
  assert.eq( { a: 1 }, t.find({ _id: 1 }, { _id: 0, a: 1 } ).next() );
  assert.eq( { _id: 0, a: 0 }, t.find( { _id: 0 }, { _id: 1, a: 1 } ).next() );
  
  // Non-simple: exclusion.
  assert.eq( { _id: 1, a: 1 }, t.find( { _id: 1 }, { b: 0 } ).next() );
  assert.eq( { _id: 0, }, t.find( { _id: 0 }, { a: 0, b: 0 } ).next() );
  
  // Non-simple: dotted fields.
  assert.eq( { b: [ { c: 1 }, { c: 2 } ] }, t.find( { _id: 0 }, { _id: 0, "b.c": 1 } ).next() );
  assert.eq( { _id: 1 }, t.find( { _id: 1 }, { "foo.bar": 1 } ).next() );
  
  // Non-simple: elemMatch projection.
  assert.eq( { _id: 1, b: [ { c: 4 } ] },
             t.find( { _id: 1 }, { b: { $elemMatch: { c: 4 } } } ).next() );
  
  // Non-simple: $returnKey.
  assert.eq( { _id: 1 }, t.find( { _id: 1 } )._addSpecial( "$returnKey", true ).next() );
  
  // Non-simple: $returnKey overrides other projections.
  assert.eq( { _id: 1 }, t.find( { _id: 1 }, { a: 1 } )._addSpecial( "$returnKey", true ).next() );
  