Dissolve polygon by attribute or featurecollection by property in Google Earth Engine?

Question

I would like to dissolve a polygon/featurecollection in GEE to run some statistics by the country level. I have tried union and dissolve and cannot find a solution. Union dissolves everything to get the boundary of all polygons. How can I dissolve by a property in GEE? It would be similar to dissolve by attribute in ArcMap/QGIS.

//Purpose: Dissolve featurecollection by a property 

//Name WDPA variable
var WDPA = ee.FeatureCollection("WCMC/WDPA/current/polygons");
print(WDPA.first(), 'WDPA'); //28 properties
Map.addLayer(WDPA, {}, 'WDPA'); //adds featurecollection to map

//Union using the property
var WDPA_union = table.union('ISO3'); //ERROR: Invalid argument specified for ee.Number(): ISO3
print(WDPA_union.first(), 'WDPA Union by ISO');
Map.addLayer(WDPA_union, {}, 'WDPA Union by ISO');

//Dissolve using the property
var WDPA_dissolve = WDPA_ISO.dissolve('ISO3'); //ERROR: Cannot read property 'dissolve' of undefined
print(WDPA_dissolve.first(), 'WDPA Dissolve by ISO');
Map.addLayer(WDPA_dissolve, {}, 'WDPA Dissolve by ISO');

link code

Answer 1

Yes that is possible. First make a list of all property values which are present in your feature collection. Then filter the featurecollection on all the features with a similar value for that property, and union only that features.

//// ---- Union using the property ---- \\\\

// first get a list of all posible property values
var propVals = ee.List(WDPA.aggregate_array(nameProperty)).distinct();
// then make a feature the union of all features having the same propVal
var unionByProp = ee.FeatureCollection(propVals.map(function(propVal){
  var tempFC = WDPA.filter(ee.Filter.eq(nameProperty, propVal));
  var unionFC = tempFC.union(maxError); // specifying a max error overcomes issues with features of diff projection
  // cast the featureCollection (output union()) to a single feature
  return ee.Feature(unionFC.first()).set(nameProperty, propVal);
}));

See the link for a full working example. Note that for printing, you will need to limit the size of this feature collection. Export the result if printing a subset satisfied your needs.