Aggregating and Annotating Objects
Overview
The Django web framework includes a default object-relational mapping layer (ORM) for interacting with data from relational databases such as SQLite, PostgreSQL, and MySQL. It is an API that Django uses and allows us to add, delete, and modify a query object. ORM stands for Object Relational Mapping. An object-relational mapper bridges the gap between relational databases and object-oriented programming languages without the need for SQL queries.
Introduction
You must have come across a situation where you have to find the total price of the materials you purchased. Similarly, when a group of records or a query set is defined, a summary of all these items is required. Well, aggregating and annotating objects refers to summing up or putting together the values. Annotation of an object creates a separate summary for each object in a queryset. Aggregation of an object is to generate summary values over an entire QuerySet.
Annotate in Django
Annotation of an object creates a separate summary for each object in a queryset. It is often used to obtain a summary of a particular object. Below is the syntax for annotation.
In the above syntax,AggregateFunction is nothing but the functions like avg for finding the average,sum for finding the sum, etc.
Example of Annotate in Django
Let us consider an example. First, we create a model.
Register the model in the admin.py file.
In the interactive console, after importing the models, let us add some records.
Now we want to know the total number of chapters in the book. Therefore, we use the annotated syntax. First, we import Count then we take an output variable chap_count in the syntax model_name.objects.annotate(), then we use "count('bookdata')". To get the count we use the output variable along with the number of the records in the model book (the starting index of the records stored is 0) along with the variable which stores the count in the annotated syntax.
Aggregate in Django
A need to find the sum of a particular column is required, when dealing with a set of data. Therefore, we come across the term aggregate, which means to form a whole by combining separate elements. We, in aggregate, combine the records of the fields to get a summary of the query set. Given below is the syntax.
Example of aggregate in Django
Considering the example we took in the above section, let us find the price of the book. We use the 'sum' aggregate function for the same.
To find the book with a maximum and minimum amount, we use the 'max' and 'min' aggregate functions.
To find the average of the price, we use 'avg' aggregate function.
Difference between Django's Annotate and Aggregate Methods
Annotate(), unlike aggregate(), is not a terminal clause. The annotate() clause returns a QuerySet. Annotations are inherently linked to individual queryset items. Aggregate result (summary) values across an entire QuerySet.
Conclusion
Hello developer!! I am sure by now you must have learned what aggregating and annotating objects are. Let us summarize what we have learned so far
- Django allows us to add, delete, modify and query objects, using an API called ORM. ORM stands for Object Relational Mapping.
- Annotation of an object creates a separate summary for each object in a queryset.
- Aggregation of an object is to generate summary values over an entire querySet.