Queryset update django

Show 3 more comments. Active Oldest Votes. There are several key differences. Ok, here's the example code. Manager, self. Community Bot 1 1 1 silver badge. See docs. Add a comment. Both looks similar, but there are some key points: save will trigger any overridden Model. Inti 3, 1 1 gold badge 26 26 silver badges 31 31 bronze badges.

FallenAngel FallenAngel AvinashRaj there still is a risk though between those 2 line for the db data to change. But it is reduced now.

Roy Roy 1 1 gold badge 2 2 silver badges 10 10 bronze badges. If you want to update multiple fields at the same time, say from a dict for a single object instance you can do something like: obj.

Class myModel models. DateField def save self : if not self. David David 1 1 1 bronze badge. Sign up or log in Sign up using Google. Sign up using Facebook. True if the QuerySet is ordered — i. False otherwise. The query parameter to QuerySet exists so that specialized query subclasses can reconstruct internal query state. The value of the parameter is an opaque representation of that query state and is not part of a public API.

Django provides a range of QuerySet refinement methods that modify either the types of results returned by the QuerySet or the way its SQL query is executed. Returns a new QuerySet containing objects that match the given lookup parameters. Returns a new QuerySet containing objects that do not match the given lookup parameters. Annotates each object in the QuerySet with the provided list of query expressions. An expression may be a simple value, a reference to a field on the model or any related models , or an aggregate expression averages, sums, etc.

Each argument to annotate is an annotation that will be added to each object in the QuerySet that is returned. The aggregation functions that are provided by Django are described in Aggregation Functions below. Annotations specified using keyword arguments will use the keyword as the alias for the annotation. Anonymous arguments will have an alias generated for them based upon the name of the aggregate function and the model field that is being aggregated.

Only aggregate expressions that reference a single field can be anonymous arguments. Everything else must be a keyword argument. For example, if you were manipulating a list of blogs, you may want to determine how many entries have been made in each blog:. For an in-depth discussion of aggregation, see the topic guide on Aggregation. Same as annotate , but instead of annotating objects in the QuerySet , saves the expression for later reuse with other QuerySet methods. This is useful when the result of the expression itself is not needed but it is used for filtering, ordering, or as a part of a complex expression.

Not selecting the unused value removes redundant work from the database which should result in better performance. For example, if you want to find blogs with more than 5 entries, but are not interested in the exact number of entries, you could do this:. To use aliased expression with other methods e. Ascending order is implied. To order randomly, use "? To order by a field in a different model, use the same syntax as when you are querying across model relations.

For example, since the Blog model has no default ordering specified:. You can also order by query expressions by calling asc or desc on the expression:. Be cautious when ordering by fields in related models if you are also using distinct. See the note in distinct for an explanation of how related model ordering can change the expected results. It is permissible to specify a multi-valued field to order the results by for example, a ManyToManyField field, or the reverse relation of a ForeignKey field.

Thus, take care when using multi-valued field to order the results. If not, make sure the results are what you expect. With respect to case-sensitivity, Django will order results however your database backend normally orders them.

You can order by a field converted to lowercase with Lower which will achieve case-consistent ordering:. You can tell if a query is ordered or not by checking the QuerySet. Ordering is not a free operation. Each field you add to the ordering incurs a cost to your database. Each foreign key you add will implicitly include all of its default orderings as well. A particular ordering is guaranteed only when ordering by a set of fields that uniquely identify each object in the results.

Calling reverse a second time restores the ordering back to the normal direction. Note that this is not quite the same as slicing from the end of a sequence in Python. The above example will return the last item first, then the penultimate item and so on.

If we had a Python sequence and looked at seq[] , we would see the fifth-last item first. Also, note that reverse should generally only be called on a QuerySet which has a defined ordering e. If no such ordering is defined for a given QuerySet , calling reverse on it has no real effect the ordering was undefined prior to calling reverse , and will remain undefined afterward.

This eliminates duplicate rows from the query results. By default, a QuerySet will not eliminate duplicate rows. In practice, this is rarely a problem, because simple queries such as Blog. This can sometimes lead to unexpected results when used in conjunction with distinct. If you order by fields from a related model, those fields will be added to the selected columns and they may make otherwise duplicate rows appear to be distinct. The moral here is that if you are using distinct be careful about ordering by related models.

Similarly, when using distinct and values together, be careful when ordering by fields not in the values call. For a normal distinct call, the database compares each field in each row when determining which rows are distinct. For a distinct call with specified field names, the database will only compare the specified field names. For example, if the Blog model defined an ordering by name :.

Returns a QuerySet that returns dictionaries, rather than model instances, when used as an iterable. Each of those dictionaries represents an object, with the keys corresponding to the attribute names of model objects. This example compares the dictionaries of values with the normal model objects:. You can use built-in and custom lookups in ordering.

An aggregate within a values clause is applied before other arguments within the same values clause. If you need to group by another value, add it to an earlier values clause instead. When using values together with distinct , be aware that ordering can affect the results. See the note in distinct for details.

If you use a values clause after an extra call, any fields defined by a select argument in the extra must be explicitly included in the values call. Any extra call made after a values call will have its extra selected fields ignored.

Combining transforms and aggregates requires the use of two annotate calls, either explicitly or as keyword arguments to values. As above, if the transform has been registered on the relevant field type the first annotate can be omitted, thus the following examples are equivalent:. This is your chance to really flaunt your individualism. Because ManyToManyField attributes and reverse relations can have multiple related rows, including these can have a multiplier effect on the size of your result set.

This will be especially pronounced if you include multiple such fields in your values query, in which case all possible combinations will be returned. This is similar to values except that instead of returning dictionaries, it returns tuples when iterated over. If you only pass in a single field, you can also pass in the flat parameter.

If True , this will mean the returned results are single values, rather than one-tuples. An example should make the difference clearer:. It is an error to pass in flat when there is more than one field.

Using a named tuple may make use of the results more readable, at the expense of a small performance penalty for transforming the results into a named tuple. A common need is to get a specific field value of a certain model instance. For example, notice the behavior when querying across a ManyToManyField :. Authors with multiple entries appear multiple times and authors without any entries have None for the entry headline. Similarly, when querying a reverse foreign key, None appears for entries not having any author:.

Returns a QuerySet that evaluates to a list of datetime. Each datetime. This specifies how to order the results. Indeed, a given datetime has different representations depending on the time zone in use. This parameter must be a datetime. Deprecated since version 4. This function performs time zone conversions directly in the database.

As a consequence, your database must be able to interpret the value of tzinfo. This translates into the following requirements:. Calling none will create a queryset that never returns any objects and no query will be executed when accessing the results. Returns a copy of the current QuerySet or QuerySet subclass. This can be useful in situations where you might want to pass in either a model manager or a QuerySet and do further filtering on the result.

When a QuerySet is evaluated , it typically caches its results. If the data in the database might have changed since a QuerySet was evaluated, you can get updated results for the same query by calling all on a previously evaluated QuerySet. In such cases, you must use the column names from the first QuerySet in QuerySet methods applied to the resulting QuerySet. Further, databases place restrictions on what operations are allowed in the combined queries.

See union for some restrictions. These querysets are equivalent:. This will follow all non-null foreign keys it can find - nullable foreign keys must be specified. This is not recommended in most cases as it is likely to make the underlying query more complex, and return more data, than is actually needed.

Returns a QuerySet that will automatically retrieve, in a single batch, related objects for each of the specified lookups. It also supports prefetching of GenericRelation and GenericForeignKey , however, it must be restricted to a homogeneous set of results. For example, prefetching objects referenced by a GenericForeignKey is only supported if the query is restricted to one ContentType.

The problem with this is that every time Pizza. This implies a self. That is, all the relevant toppings will have been fetched in a single query, and used to make QuerySets that have a pre-filled cache of the relevant results; these QuerySets are then used in the self.

Note that the result cache of the primary QuerySet and all specified related objects will then be fully loaded into memory. This changes the typical behavior of QuerySets , which normally try to avoid loading all objects into memory before they are needed, even after a query has been executed in the database. Remember that, as always with QuerySets , any subsequent chained methods which imply a different database query will ignore previously cached results, and retrieve data using a fresh database query.

So, if you write the following:. So use this feature with caution! Also, if you call the database-altering methods add , remove , clear or set , on related managers , any prefetched cache for the relation will be cleared. You can also use the normal join syntax to do related fields of related fields.

Suppose we have an additional model to the example above:. This will prefetch all pizzas belonging to restaurants, and all toppings belonging to those pizzas.

This will result in a total of 3 database queries - one for the restaurants, one for the pizzas, and one for the toppings. This will fetch the best pizza and all the toppings for the best pizza for each restaurant. This will normally happen with foreign key relationships.

Save my name, email, and website in this browser for the next time I comment. Skip to content Pythoncoders. Django update queryset in a single line Posted by allwinraju July 19, View this gist on GitHub. Ask Question. Asked today. Active today. Viewed 11 times. The result of my 2 querysets are not equal, but the 2 variables are. Add a comment.

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