A queryset is an object that represents a database query using a specific Model. It is lazy, meaning that it does not hit the database until you iterate over its matching rows (model instances). To create a base queryset for a model class, use:
The `filter` and `exclude` methods are used for filtering the matching instances. Calling these methods returns a new queryset instance, with the added conditions. For example:
"((first_name = 'Ciaran' AND last_name = 'Carver') OR height <= 1.8) AND (NOT (first_name = 'David'))"
Note that Python's bitwise operators (`&`, `|`, `~`, `^`) have higher precedence than comparison operators, so always use parentheses when combining these two types of operators in an expression. Otherwise the resulting SQL might be different than what you would expect.
### Using `IN` and `NOT IN`
Filtering queries using ClickHouse's `IN` and `NOT IN` operators requires using the `isIn` and `isNotIn` functions (trying to use Python's `in` keyword will not work!).
For example:
```python
# Is it Monday, Tuesday or Wednesday?
F.isIn(F.toDayOfWeek(F.now()), [1, 2, 3])
# This will not work:
F.toDayOfWeek(F.now()) in [1, 2, 3]
```
In case of model fields, there is a simplified syntax:
The `isIn` and `isNotIn` functions expect either a list/tuple of values, or another queryset (a subquery). For example if we want to select only people with Irish last names:
Prior to version 2 of the ORM, filtering conditions were limited to a predefined set of operators, and complex expressions were not supported. This old syntax is still available, so you can use it alongside or even intermixed with new-style functions and expressions.
The old syntax uses keyword arguments to the `filter` and `exclude` methods, that are built as `<fieldname>__<operator>=<value>` (two underscores separate the field name from the operator). In case no operator is given, `eq` is used by default. For example:
When some of the model fields aren't needed, it is more efficient to omit them from the query. This is especially true when there are large fields that may slow the query down. Use the `only` method to specify which fields to retrieve:
The `paginate` method returns a `namedtuple` containing the following fields:
-`objects` - the list of objects in this page
-`number_of_objects` - total number of objects in all pages
-`pages_total` - total number of pages
-`number` - the page number, starting from 1; the special value -1 may be used to retrieve the last page
-`page_size` - the number of objects per page
Note that you should use `QuerySet.order_by` so that the ordering is unique, otherwise there might be inconsistencies in the pagination (such as an instance that appears on two different pages).
Aggregation
-----------
It is possible to use aggregation functions over querysets using the `aggregate` method. The simplest form of aggregation works over all rows in the queryset:
The returned row or rows are no longer instances of the base model (`Person` in this example), but rather instances of an ad-hoc model that includes only the fields specified in the call to `aggregate`.
You can pass fields from the model that will be included in the query. By default, they will be also used in the GROUP BY clause. For example to count the number of people per last name you could do this:
If you would like to control the GROUP BY explicitly, use the `group_by` method. This is useful when you need to group by a calculated field, instead of a field that exists in the model. For example, to count the number of people born on each weekday:
SELECT toDayOfWeek(birthday) AS weekday, count() AS num
FROM person
GROUP BY weekday
After calling `aggregate` you can still use most of the regular queryset methods, such as `count`, `order_by` and `paginate`. It is not possible, however, to call `only` or `aggregate`. It is also not possible to filter the aggregated queryset on calculated fields, only on fields that exist in the model.
