Google Earth Engine: cannot get the properties which are set in the map function

Question

I tried to set new properties in a map function, but I failed to get these properties outside the map function. The error is:

    ComputedObject (Error)
    Collection.first: Error in map(ID=0):
    Feature.intersection: Parameter 'right' is required.

And the code is:

// double map over the polygons
var mapped = vectors.map(function(feat1){
  feat1 = ee.Feature(feat1);
  var mapped1 = polygons.map(function(feat2){
    feat2 = ee.Feature(feat2);
    var intersection = feat2.intersection(feat1, ee.ErrorMargin(1));
    return ee.Feature(intersection).set({'Intersect': intersection.area().divide(1000 * 1000),'date':'2016-01-01'})
  })
  return mapped1
}).flatten().filter(ee.Filter.gt('Intersect', 0));

var test = mapped.first().get('date');
print(test);

As I set new properties "Intersect" and "date", but I cannot get any of them when I print it.

The full code can be found at: https://code.earthengine.google.com/e31d1380d8499df42194fa2a5afa08f1

The test code is written by Rodrigo E. Principe https://gis.stackexchange.com/users/23470/rodrigo-e-principe

Answer 1

Don't turn things into lists unnecessarily. I also don't think this is a bug. One of your intersections is null, so that's why you can't set properties on it. Here's a good way to solve that, by allowing map() to return nulls (and drop them):

var mapped = vectors.map(function(feat1){
  feat1 = ee.Feature(feat1);
  var mapped1 = polygons.map(function(feat2){
    feat2 = ee.Feature(feat2);
    var intersection = feat2.intersection(feat1, ee.ErrorMargin(1));
    return intersection
  }, true) // Note the boolean flag.  See map() docs for details.
  return mapped1
}).flatten();

// Now set properties on non-null features.
mapped = mapped.map(function(intersection) {
  return intersection.set({
    'Intersect': intersection.area().divide(1000 * 1000),
    'date': '2016-01-01'})
});

print(mapped.first());
var aaa = mapped.first().get('date');
print(aaa);

https://code.earthengine.google.com/6162e6ea95efa8b00b29b2e8514e5735

Answer 2

The fact that print(mapped.first()) works but print(mapped.first().get('date')) fails even though the former shows a date property, and the error message is about steps that precede fetching the date property, suggests that this is a bug. I will report it.

Answer 3

I find a temporally useful way to solve this problem. You can transform the Feature type to List type and use the get function of List type to obtain what you want. The code looks like:

// double map over the polygons
var mapped = vectors.map(function(feat1){
  feat1 = ee.Feature(feat1);
  var mapped1 = polygons.map(function(feat2){
    feat2 = ee.Feature(feat2);
    var intersection = feat2.intersection(feat1, ee.ErrorMargin(1));
    return ee.Feature(intersection).set({'Intersect': intersection.area().divide(1000 * 1000),'date':'2016-01-01'})
  })
  return mapped1
}).flatten().filter(ee.Filter.gt('Intersect', 0));

var lists = mapped.toList(3); // 3 is temporally set, and it should be the number of your features
var ff = ee.Feature(lists.get(0));  // get the first feature
print(ff.get('Intersect'));      // This will work.