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The placement of only in a sentence has been a source of studious commentary since the 18th century, most of it intended to prove by force of argument that prevailing standard usage is wrong. After 200 years of preachment the following observations may be made: the position of only in standard spoken English is not fixed, since ambiguity is avoided through sentence stress; in casual prose that keeps close to the rhythms of speech only is often placed where it would be in speech; and in edited and more formal prose only tends to be placed immediately before the word or words it modifies.
Internally, a QuerySet can be constructed, filtered, sliced, and generallypassed around without actually hitting the database. No database activityactually occurs until you do something to evaluate the queryset.
Also note that even though slicing an unevaluated QuerySet returnsanother unevaluated QuerySet, modifying it further (e.g., addingmore filters, or modifying ordering) is not allowed, since that does nottranslate well into SQL and it would not have a clear meaning either.
If you pickle a QuerySet, this will force all the results to be loadedinto memory prior to pickling. Pickling is usually used as a precursor tocaching and when the cached queryset is reloaded, you want the results toalready be present and ready for use (reading from the database can take sometime, defeating the purpose of caching). This means that when you unpickle aQuerySet, it contains the results at the moment it was pickled, ratherthan the results that are currently in the database.
If you only want to pickle the necessary information to recreate theQuerySet from the database at a later time, pickle the query attributeof the QuerySet. You can then recreate the original QuerySet (withoutany results loaded) using some code like this:
Pickles of QuerySets are only valid for the version of Django thatwas used to generate them. If you generate a pickle using Djangoversion N, there is no guarantee that pickle will be readable withDjango version N+1. Pickles should not be used as part of a long-termarchival strategy.
The lookup parameters (**kwargs) should be in the format described inField lookups below. Multiple parameters are joined via AND in theunderlying SQL statement, and the whole thing is enclosed in a NOT().
Annotates each object in the QuerySet with the provided list of queryexpressions. An expression may be a simple value, areference to a field on the model (or any related models), or an aggregateexpression (averages, sums, etc.) that has been computed over the objects thatare related to the objects in the QuerySet.
Annotations specified using keyword arguments will use the keyword asthe alias for the annotation. Anonymous arguments will have an aliasgenerated for them based upon the name of the aggregate function andthe model field that is being aggregated. Only aggregate expressionsthat reference a single field can be anonymous arguments. Everythingelse must be a keyword argument.
Same as annotate(), but instead of annotating objects in theQuerySet, saves the expression for later reuse with other QuerySetmethods. This is useful when the result of the expression itself is not neededbut 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 whichshould result in better performance.
filter() and order_by() can take expressions directly, butexpression construction and usage often does not happen in the same place (forexample, QuerySet method creates expressions, for later use in views).alias() allows building complex expressions incrementally, possiblyspanning multiple methods and modules, refer to the expression parts by theiraliases and only use annotate() for the final result.
The result above will be ordered by pub_date descending, then byheadline ascending. The negative sign in front of "-pub_date" indicatesdescending order. Ascending order is implied. To order randomly, use "?",like so:
To order by a field in a different model, use the same syntax as when you arequerying across model relations. That is, the name of the field, followed by adouble underscore (__), followed by the name of the field in the new model,and so on for as many models as you want to join. For example:
Here, there could potentially be multiple ordering data for each Event;each Event with multiple children will be returned multiple timesinto the new QuerySet that order_by() creates. In other words,using order_by() on the QuerySet could return more items than youwere working on to begin with - which is probably neither expected noruseful.
Also, note that reverse() should generally only be called on a QuerySetwhich has a defined ordering (e.g., when querying against a model which definesa default ordering, or when using order_by()). If no such ordering isdefined for a given QuerySet, calling reverse() on it has no realeffect (the ordering was undefined prior to calling reverse(), and willremain undefined afterward).
Keep in mind that order_by() uses any default related model orderingthat has been defined. You might have to explicitly order by the relation_id or referenced field to make sure the DISTINCT ON expressionsmatch those at the beginning of the ORDER BY clause. For example, ifthe Blog model defined an ordering byname:
An aggregate within a values() clause is applied before other argumentswithin the same values() clause. If you need to group by another value,add it to an earlier values() clause instead. For example:
If you only pass in a single field, you can also pass in the flatparameter. If True, this will mean the returned results are single values,rather than 1-tuples. An example should make the difference clearer:
This function performs time zone conversions directly in the database.As a consequence, your database must be able to interpret the value oftzinfo.tzname(None). This translates into the following requirements:
When a QuerySet is evaluated, ittypically caches its results. If the data in the database might have changedsince a QuerySet was evaluated, you can get updated results for the samequery by calling all() on a previously evaluated QuerySet.
This implies a self.toppings.all() for each Pizza; now each timeself.toppings.all() is called, instead of having to go to the database forthe items, it will find them in a prefetched QuerySet cache that waspopulated in a single query.
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 relevantresults; these QuerySets are then used in the self.toppings.all() calls.
Note that there is no mechanism to prevent another database query from alteringthe items in between the execution of the primary query and the additionalqueries, which could produce an inconsistent result. For example, if aPizza is deleted after the primary query has executed, its toppings willnot be returned in the additional query, and it will seem like the pizza has notoppings:
Note that the result cache of the primary QuerySet and all specified relatedobjects will then be fully loaded into memory. This changes the typicalbehavior of QuerySets, which normally try to avoid loading all objects intomemory before they are needed, even after a query has been executed in thedatabase.
Remember that, as always with QuerySets, any subsequent chained methodswhich imply a different database query will ignore previously cachedresults, and retrieve data using a fresh database query. So, if you writethe following:
This will prefetch all pizzas belonging to restaurants, and all toppingsbelonging 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.
Since the prefetch is executed after the main query (which includes the joinsneeded by select_related), it is able to detect that the best_pizzaobjects have already been fetched, and it will skip fetching them again.
One difference to note when using prefetch_related is that objects createdby a query can be shared between the different objects that they are related toi.e. a single Python model instance can appear at more than one point in thetree of objects that are returned. This will normally happen with foreign keyrelationships. Typically this behavior will not be a problem, and will in factsave both memory and CPU time.
While prefetch_related supports prefetching GenericForeignKeyrelationships, the number of queries will depend on the data. Since aGenericForeignKey can reference data in multiple tables, one query per tablereferenced is needed, rather than one query for all the items. There could beadditional queries on the ContentType table if the relevant rows have notalready been fetched.
When using multiple databases, Prefetch will respect your choice ofdatabase. If the inner query does not specify a database, it will use thedatabase selected by the outer query. All of the following are valid:
This will raise a ValueError because of the attempt to redefine thequeryset of a previously seen lookup. Note that an implicit queryset wascreated to traverse 'pizzas' as part of the 'pizzas__toppings'lookup.
This is an old API that we aim to deprecate at some point in the future.Use it only if you cannot express your query using other queryset methods.If you do need to use it, please file a ticket using the QuerySet.extrakeywordwith your use case (please check the list of existing tickets first) sothat we can enhance the QuerySet API to allow removing extra(). We areno longer improving or fixing bugs for this method.
If you need to order the resulting queryset using some of the newfields or tables you have included via extra() use the order_byparameter to extra() and pass in a sequence of strings. Thesestrings should either be model fields (as in the normalorder_by() method on querysets), of the formtable_name.column_name or an alias for a column that you specifiedin the select parameter to extra().
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