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mongoengine: get data and store in another DB

it is my first time to use mongoengine, I know the ORM concept.
I want to know may I get the data in one DB and using the same object to store in another DB??
class User(Document):
email = EmailField(required=True, unique= True)
salary = IntField(require=True)
connect(alias='default',db='tumblelog')
connect(alias='testdb',db='testdb')
users = User.objects
with switch_db(User,'testdb') as User:
for user in users:
User(email=user.email,salary=user.email).save # it works
user.save() #doesn't works

I've found out that by using ORM to get a data from one DB, it will create an object unique to the DB you get it from. You won't be able to use the same object to store it into another DB.
What I would suggest is that you initialise an empty object, for example User, for the other DB and fill it with values from the original object and then store it.
You might want to have a look at this question. Sequelize: Using Multiple Databases

Is it possible to query a query object in Django?

I am trying to query my postgres database from django, the query I'm using is
s = Booking.objects.all().filter(modified_at__range=[last_run, current_time], coupon_code__in=l)
Now I am changing this object of mine in some ways in my script, and not saving it to the database. What I want to know is that, is it possible to query this object now?
say, I changed my variable as
s.modified_at = '2016-02-22'
Is it still possible to query this object as:
s.objects.all()
or something similar?

The QueryManager is Django's interface to the database (ORM). By definition this means you can only query data that has been stored in the database.
So, in short: "no". You cannot do queries on unsaved data.
Thinking about why you are even asking this, especially looking at the example using "modified_at": why do you not want to save your data?
(You might want to use auto_now=True for your "modified_at" field, btw.)

You could do something like this:
bookings = Booking.objects.all().filter(modified_at__range=[last_run, current_time], coupon_code__in=l)
for booking in bookings:
booking.modified_at = 'some value'
booking.save() # now booking object will have the updated value

How to serialize and deserialize Django ORM query (not queryset)?

My use case is that I need to store queries in DB and retrieve them from time to time and evaluate. Thats needed for mailing-app where every user can subscribe to a web-site content selected by individually customized query.
Most basic solution is to store raw SQL and use it with RawQuerySet. But I wonder is there better solutions?

At first glance, it is really dangerous to hand out query building job to others, since they can do anything (even delete all your data in your database or drop entire table etc.)
Even you let them build a specific part of the query, it is still open to Sql Injection. If it is ok for all those dangers, then you may try the following.
This is and old script I used and let users set a specific part of the query. Basics are using string.Template and eval (the evil part)
Define your Model:
class SomeModel(Model):
usr = ForeingKey(User)
ct = ForeignKey(ContentType) # we will choose related DB table with this
extra_params = TextField() # store extra filtering criteria in here
Lets execute all queries belongs to a user. Say we have a User query with extra_params is_staff and 'username__iontains'
usr: somebody
ct: User
extra_params: is_staff=$stff_stat, username__icontains='$uname'
$ defines placeholders in extra_params
from string import Template
for _qry in SomeModel.objects.filter(usr='somebody'): # filter somebody's queries
cts = Template(_qry.extra_params) # take extras with Template
f_cts = cts.substitute(stff_stat=True, uname='Lennon') # sustitute placeholders with real time filtering values
# f_cts is now `is_staff=True, username__icontains='Lennon'`
qry = Template('_qry.ct.model_class().objects.filter($f_cts)') # Now, use Template again to place our extras into a django `filter` query. We also select related model in here with `_qry.ct.model_class()`
exec_qry = qry.substitute(f_cts=f_cts)
# now we have `User.objects.filter(is_staff=True, username__icontains='Lennon')
query = eval(exec_qry) # lets evaluate it!
If you have all relted imports done,then you an use Q or any other query building option in your extra_params. Also You can use other methods to form Create or Update queries.
You can read more about Template form there. But as I said. It is REALLY DANGEROUS to give a such option to other users.
Also you may need to read about Django Content Type
Update: As #GillBates mentioned, you can use a dictonary structure to create the query. In this case, you will not need Template anymore. You can use json for such data transfer (or any other if you wish). Assuming you use json to get the data from an outer source following code is a scratch that uses some variables from the upper code block.
input_data : '{"is_staff"=true, "username__icontains"="Lennon"}'
import json
_data = json.loads(input_data)
result_set = _qry.ct.model_class().objects.filter(**_data)

According to your answer,
User passes some content-specific parameters into a form, then view function, that recieves POST, constructs query
one option is to store parameters (pickle'd or json'ed, or in a model) and reconstruct query with regular django means. This is somewhat more robust solution, since it can handle some datastructure changes.

You could create a new model user_option and store the selections in this table.
From your question, it's hard to determine whether it is a better solution, but it would make your user's choices more explicit in your data structure.

unique python/django issue with variable column/field name in form

I have quite unique problem with django.
Im providing website users interface for editing large data. Each row on this data represents a row in database. Or one object of certain Type.
Users click on cells in the table and form opens where they can edit this fields/column value.
In essence it works like this:
1) based on where user clicks, query is sent to server containting object id and the field that he is editing.
2) based on this information form is created on the fly:
class FieldEditorForm(forms.ModelForm):
class Meta:
model = MyObject
fields = ['id', field ]
Notice the field there is Variable not name of the field.
3) this field passes its own modelform validation and all is fine. in save method Model.save() is enough to update the value.
But now to the problem. Sometimes empty value is sent to server in this form. Empy value such as u'' or almost emtpty like u' '. I want to repace this with None so NULL would be saved to database.
There are two places where i could do that. In field validation modifying the cleaned_data or in form save method.
Both approaches raise unique problem as i dont know how to create variable function names.
def clean_%(field)s():
or in case of form save method
r.%(field)s = None
is what i need, but those methods dont work. So how can i create method name which is variable or set objects variable parameter to something. Is it even possible or do i have to rethink my approach there?
Alan

In the latter case, setattr(r, field + 's', None).

Mongoengine integer id, and User creating

MongoDB is using string(hash) _id field instead of integer; so, how to get classic id primary key? Increment some variable each time I create my class instance?
class Post(Document):
authors_id = ListField(IntField(required=True), required=True)
content = StringField(max_length=100000, required=True)
id = IntField(required=True, primary_key=True)
def __init__(self):
//what next?
Trying to create new user raises exception:
mongoengine.queryset.OperationError: Tried to save duplicate unique keys
(E11000 duplicate key error index: test.user.$_types_1_username_1
dup key: { : "User", : "admin" })
Code:
user = User.create_user(username='admin', email='example#mail.com',
password='pass')
user.is_superuser = True
user.save()
Why?

There is the SequenceField which you could use to provide this. But as stated incrementing id's dont scale well and are they really needed? Can't you use ObjectId or a slug instead?

If you want to use an incrementing integer ID, the method to do it is described here:
http://www.mongodb.org/display/DOCS/How+to+Make+an+Auto+Incrementing+Field
This won't scale for a vary large DB/app but it works well for small or moderate application.

1) If you really want to do it you have to override the mongoengine method saving your documents, to make it look for one document with the highest value for your id and save the document using that id+1. This will create overhead (and one additional read every write), therefore I discourage you to follow this path. You could also have issues of duplicated IDs (if you save two records at the exactly same time, you'll read twice the last id - say 1 and save twice the id 1+1 = 2 => that's really bad - to avoid this issue you'd need to lock the entire collection at every insert, by losing performances).
2) Simply you can't save more than one user with the same username (as the error message is telling you) - and you already have a user called "admin".