Your entity class must have the two primary/foreign key from each ends of the many-to-many link, in this case it’s the BookId and the TagId. Creating a class to map to the linking table Typically, you would only define the linking table if you wanted to add extra data. You can define an entity class and configure the linking table, but I will say that if you are going to do that you might as well use the indirect approach as I think it’s easier to set up and use. Direct many-to-many setup: When you want to define the linking table
WithMany() where the Tags has no navigational property back to the Books. If you only want a navigational property on one end, then you will need to use the Fluent API configure (see next section), for instance …HasMany(x => x.Tags). NOTE: The direct many-to-many relationship is only automatically configured if you have a collection navigational property on both ends. But if you want to add extra data in the linking table, say for ordering or filtering, then you either alter the direct many-to-many or use the indirect many-to-many approach. This is super simple to do – so much easier than the indirect many-to-many. And when you create your database via EF Core, then it will add the linking table for you. The setting up of the direct many-to-many relationship is done automatically (this is known as By Convention configuration in EF Core). Direct many-to-many setup – normal setup. Now we get into the detail of setting up and using both of these types of many-to-many relationships. But for the Book’s Tags (bottom left), which don’t have an order, the query is much simpler to write because EF Core will automatically add the extra SQL needed to use the hidden linking table. You can see that the Book’s Authors (top left) needs to be ordered – that Order property (a byte) is in the linking entity class. Next, let’s see the many-to-many queries and how they relate to the book display in the figure below. This shows the two many-to-many – both have a linking table, but the direct many-to-many has its linking table created by EF Core. You can skip this, but maybe having an overall view of what is going on will help you when you are looking at the detailed part you are looking at the specific part you are interested in. Let’s start by seeing the finished database and how the query works. Setting the scene – the database and the query
#EF COLLECTION CODE#
Here is link to the directory with the entity classes are in, and many of code examples comes from the Ch03_ManyToManyUpdate unit test class and Ch03_ManyToMan圜reate. NOTE: All the code you see in this article comes the companion GitHub repo to my book Entity Framework Core in Action.
Indirect many-to-many setup – configuring the linking table.When you want to define the linking table.Here are the individual summaries (with links). This allows you to put specific data in the linking table, such as an order in which you want to read them back. Indirect many-to many relationships takes more work to set up, but you can access the linking table.Direct many-to many relationships are super simple to configure and use, but by default you can’t access the linking table.Here are the links to the summaries: The overall summary is: Overall summary and links to each section summaryįor people who are in a hurry I have a ended each section with a summary. Here is an example of how it displays each book to the user – this is a fictitious book I used for many of my tests. An indirect many-to-many relationship to Author entity class, which provides an ordered list of Author’s on the book, for instance: by Dino Esposito, Andrea Saltarello.NET or Web) which allows users to pick books by their topic. A Tag holds a category (for instance: Microsoft. A direct many-to-many relationship to a Tag entity class (I refer to classes that EF Core maps to a database as entity classes).In this book I build a book selling site, called Book App, where each book has two, many-to-many relationships: I hope you find it useful.Īll the information and the code come from Chapter 2 of the second edition of my book, Entity Framework Core in Action, which cover EF Core 5. All of these got a lot of views, so I had to write a new article once EF Core 5 came out. I wrote the first article on many-to-many on EF6.x in 2014, and another many-to-many article for EF Core in 2017.
You might be mildly interested that this is the third iteration of this article. I also include the original many-to-many relationship (referred to as indirect many-to-many from now on) where you need to create the class that acts as the linking table between the two classes.
#EF COLLECTION HOW TO#
This article describes how to set this direct many-to-many relationship and why (and how) you might want to configure this direct many-to-many. Last Updated: Ma| Created: January 14, 2021ĮF Core 5 added a direct many-to-many relationship with zero configuration (hurrah!).
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