Get the list of name of fields in mongo using pymongo and mongodb shell

followings will list all fields exist in a collection in mongodb :

Pymongo: 

#Using pymongo and connection named 'db'

reduce(
    lambda all_keys, rec_keys: all_keys | set(rec_keys), 
    map(lambda d: d.keys(), db.things.find()), 
    set()
)

Mongodb Shell:

mr = db.runCommand(

{"mapreduce" : "temp",  

"map" : function() {
    for (var key in this) { emit(key, null); }
  },
  "reduce" : function(key, stuff) { return null; },
  "out": "temp" + "_keys"
}) 



db[mr.result].distinct("_id")

How to change data type of a field in mongo

Following query helps you convert the data type of a field from

• string to integer:

db.my_collection.find().forEach( function(obj) {
    obj.field-name= parseInt(obj.field-name);
    db.my_collection.save(obj);

});

• and from integer to string : 

db.my_collection.find().forEach( function(obj) {
    obj.field-name= ""+obj.field-name;
    db.my_collection.save(obj);

});

How to autoincrement id at every insert in Pymongo?

Use a separate counters collection to track the last number sequence used. The _id field contains the sequence name and the seq field contains the last value of the sequence.

1- Insert into the counters collection, the initial value for the userid:

 db.orgid_counter.insert({'_id': "userid", 'seq': 0})  

2- Create a getNextSequence function in target language(I am using python) that accepts a name of the sequence. The function uses the find_And_Modify() method to atomically increment the seq value and return the new value:

 def getNextSequence(collection,name):  
   return collection.find_and_modify(query= { '_id': name },update= { '$inc': {'seq': 1}}, new=True ).get('seq');  

3-Use this getNextSequence() function during insert() and insert new user with new generated id .

  db.users.insert({'_uid': getNextSequence(db.orgid_counter,"userid"), 'name': "Sara a"})  

You can verify the results with find():

 db.users.find()