function addMask(img){

  // grabbing the attached image
  var fmask_img = img.get('fmask');

  // Trying to access a band in the original image works
  //var b1 = img.select(["B1"]);

  // Trying to access a band in the image doesn't work
  //var fmask = fmask_img.select(["fmask"]);

  //trying to add a band from the attached image to the orignal image doesn't work
  //var newImg = img.addBands(fmask_img, ["fmask"])

  //trying to all bands from the attached image to the orignal image also doesn't work
  //var newImg = img.addBands(fmask_img, ["fmask"])

  //even trying to the origianl image to is self doesn't work
  //var newImg = img.addBands(img)

  return img // returning the unchanged image instead of newImg
}

// Load a primary collection: Landsat imagery.
var primary = ee.ImageCollection('LC8_L1T')
    .filterDate('2014-04-01', '2014-06-01')
    .filterBounds(ee.Geometry.Point(-122.092, 37.42));

// Load a secondary collection: MODIS imagery.
var secondary = ee.ImageCollection('LANDSAT/LC8_L1T_TOA_FMASK')
    .filterDate('2014-04-01', '2014-06-01')
    .filterBounds(ee.Geometry.Point(-122.092, 37.42));

// Create a filter to define a match based on the scene ID.
var sceneFilter = ee.Filter.equals({
  leftField: "LANDSAT_SCENE_ID",
  rightField: "LANDSAT_SCENE_ID"
  })

// Define the join.
var saveFirstJoin = ee.Join.saveFirst({
  matchKey: 'fmask',
  ordering: 'system:time_start',
  ascending: true
});


// Apply the join.
// This returns an image collection
var landsatFmask = saveFirstJoin.apply(primary, secondary, sceneFilter);

// Mapping a simple function where input = output returns an image collection
var fmask = landsatFmask.map(addMask);

// applying a reducer on the orignal collection works
var primary_median = primary.median()

//applying a reducer on the orignalnew collection works doesn't work
//var fmask_median = fmask.median()
//var joined_median = landsatFmask.median()


// Display the result.
// the orginal collection
print('raw:', primary)
// looks the same as the orginal, only difference is that images inside the collection have an additional property "fmask" with the joined image inside
print('Join.saveAll:', landsatFmask);
// looks the same as the joined collection, but now the band list of the images collection is empty ??
print('fmask:', fmask);

function addMask(img){

  // cast the input image
  var i = ee.Image(img);
  // grabbing the attached image and cast it as well
  var fmask_img = ee.Image(img.get('fmask'));

  //now i can add the band
  var newImg = i.addBands(fmask_img, ["fmask"])

  return newImg
}