Shulou(Shulou.com)06/01 Report--

In this issue, the editor will bring you about how to use Q () objects in Django. The article is rich in content and analyzes and narrates it from a professional point of view. I hope you can get something after reading this article.

problem

Generally speaking, we query the database in the Django program in QuerySet, such as the following code:

> Q1 = Entry.objects.filter (headline__startswith= "What") > Q2 = q1.exclude (pub_date__gte=datetime.date.today ()) > Q3 = q1.filter (pub_date__gte=datetime.date.today ())

Or combine them, for example:

> Q1 = Entry.objects.filter (headline_startswith= "What") .resume (pub_date_gte=datetime.date.today ())

As our program becomes more and more complex, the query conditions become more complex, so the simple query conditions through a filter () will cause our query to become longer and longer.

The Q () object is designed to combine these conditions.

When we need to combine conditions in a query (for example, two conditions "and" or "). We can use Q () to query the object. For example, the following code

Fromdjango.db.modelsimports Qq=Q (question_startswith= "What")

This generates a Q () object, and we can use symbols & or | to combine multiple Q () objects and pass them to filter (), exclude (), get (), and so on. When multiple Q () objects are combined, Django automatically generates a new Q (). For example, the following code combines two conditions into one

Q (question__startswith='Who') | Q (question__startswith='What')

Using the above code, you can understand it this way using SQL statements:

WHEREquestionLIKE 'Who%' ORquestionLIKE' What%'

We can use the character "~" in front of the Q () object to represent the meaning "not", such as the following code:

Q (question__startswith='Who') | ~ Q (pub_date__year=2005)

The corresponding SQL statement can be understood as:

WHEREquestionlike "Who%" ORyear (pub_date)! = 2005

In this way, we can use "&" or "|" and parentheses to group conditions into more complex query logic.

You can also pass multiple Q () objects to the query function, such as the following code:

News.objects.get (Q (question__startswith='Who'), Q (pub_date=date (2005, 5,2)) | Q (pub_date=date (2005, 5,6)

The relationship Django between multiple Q () objects is automatically understood as a "and (and)" relationship. If the above code uses SQL statements to understand, it will be:

SELECT * fromnewsWHEREquestionLIKE 'Who%' AND (pub_date =' 2005-05-02 'ORpub_date =' 2005-05-06')

The Q () object can be passed to the query function together with the keyword parameter, but it is important to note that the Q () object should be placed in front of the keyword parameter, see the following code

# correct practice News.objects.get (Q (pub_date=date (2005, 5, 2)) | Q (pub_date=date (2005, 5, 6)), question__startswith='Who') # wrong practice, the code puts the keyword parameter in front of the Q () object. News.objects.get (question__startswith='Who', Q (pub_date=date (2005, 5, 2)) | Q (pub_date=date (2005, 5, 6) the above is what the editor shares about how to use Q () objects in Django. If you happen to have similar doubts, please refer to the above analysis for understanding. If you want to know more about it, you are welcome to follow the industry information channel.

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