[bug] wired observe behavior

[Brooooooklyn]

Code to reproduce:

var schemaBuilder = lf.schema.create('todo', 1);

schemaBuilder.createTable('Item').
    addColumn('id', lf.Type.INTEGER).
    addColumn('name', lf.Type.STRING).
    addPrimaryKey(['id']);
    
var db;
var item;
schemaBuilder.connect().then(function(database) {
  db = database;
  item = db.getSchema().table('Item');
  var rows = [
    item.createRow({'id': 1, 'name': 'foo'}),
    item.createRow({'id': 2, 'name': 'bar'}),
  ];
  
  return db.insertOrReplace().into(item).values(rows).exec();
}).then(function() {
  // case one
  // observe a query, observe callback would not be fired
  var selectQuery = db.select().from(item);
  db.observe(selectQuery, function(changes) {
    selectQuery.exec().then(values => {
      console.log('just observe: ', values);
    });
  });
}).then(function() {
  // case two
  // observe a query and exec the query
  // observe callback would be fired
  var selectQuery = db.select().from(item);
  db.observe(selectQuery, function(changes) {
     selectQuery.exec().then(values => {
       console.log('empty results: ', values);
     });
  });
  // would triggle changes
  return selectQuery.exec();
}).then(function() {
  // case three
  // observe a query and exec the query
  // but the results of the query is an empty array
  // observe callback would not be fired
   var selectQuery = db.select().from(item).where(item.id.gt(2));
   db.observe(selectQuery, function(changes) {
     selectQuery.exec().then(values => {
       console.log('empty results: ', values);
     });
   });
   // would not triggle changes
   return selectQuery.exec().then(results => {
     console.log('empty: ', results)
   })
})

see: https://jsfiddle.net/6t3fofwm/11/

I'm confused which case is working as expected.

[freshp86]

case 1 and case 2 seeems like a duplicate of #209?

[Brooooooklyn]

in #209 , observe callback is fired after update or delete query.
my problem is, in the case2 :

db.observe(query, callback)
query.exec()

observe callback is fired by query.exec(), is this a expected behavior?
the difference between #209 and this case is, nothing updated after the query was observed, but the callback is fired.