chapter two

2 Querying the database

 

This chapter covers

  • Modelling three main types of database relationships
  • Creating and changing a database via migration
  • Defining and creating an application DbContext
  • Loading related data
  • Splitting complex queries into subqueries

This chapter is all about using EF Core for reading, called querying, the database. You’ll create a database that contains the three main types of database relationships found in EF Core. Along the way, you’ll learn to create and change a database’s structure via EF Core.

Next you’ll learn how to access a database via EF Core, reading data from the database tables. You’ll explore the basic format of EF Core queries before looking at various approaches to loading related data with the main data; for instance, loading the author with the book from chapter 1.

After learning the ways to load related data, you’ll start to build the more complex queries needed to make a book-selling site work. This covers sorting, filtering, and paging, plus approaches to combine each of these separate query commands to create one composite database query.

TIP

I use unit tests to ensure that what I write in this book is correct. You might like to look at/run these unit tests, as they may help you understand what is going on. You can find them in the associated GitHub repo https://github.com/JonPSmith/EfCoreinAction-SecondEdition – look at the README file in the repo for information on where to find the unit tests and how to run them.

2.1   Setting the scene—our book-selling site

2.1.1   The book app’s relational database

2.1.2   Other relationship types not covered in this chapter

2.1.3   The database showing all the tables

2.1.4   The classes that EF Core maps to the database

2.2   Creating the application’s DbContext

2.2.1   Defining the application’s DbContext: EfCoreContext

2.2.2   Creating an instance of the application’s DbContext

2.2.3   Creating a database for your own application

2.3   Understanding database queries

2.3.1   Application’s DbContext property access

2.3.2   A series of LINQ/EF Core commands

2.3.3   The execute command

2.3.4   The two types of database queries

2.4   Loading related data

2.4.1   Eager loading: loading relationships with the primary entity class

2.4.2   Explicit loading: loading relationships after the primary entity class

2.4.3   Select loading: loading specific parts of primary entity class and any relationships

2.4.4   Lazy loading: loading relationships as required

2.5   Using client vs. server evaluation: adapting data at the last stage of a query

2.6   Introducing the architecture of the book app

2.7   Building complex queries

2.8   Adding sorting, filtering, and paging