Dapper - 使用一对多和多对多关系的数据库查询创建对象

Question

I have an Sqlite database, for a which a partial schema is:

In the code, classes representing the data are as follows (simplified):

public Book : Item
{
    public ICollection<Tag> Tags { get; set; }
    public ICollection<Author> Authors { get; set; }
    public Publisher publisher { get; set; }
    // other properties...
}

public class Publisher
{
    public ICollection<Book> Books { get; set; }
    // other properties...
}

public class Author
{
    public ICollection<Author> Authors { get; set; }
}

public class Tag
{
    public ICollection<Item> Items { get; set; }
}

I need to return an IEnumerable<Book> , with the Book objects containing all Author , Publisher and Tag data for each Book . Currently, I have the following code, which solves the problem of getting publisher data:

public IEnumerable<Book> GetAllBooks()
{
    using (var db = new SqliteConnection(connectionString)
    {
        var sql = "SELECT * FROM Books as B " +
                  "INNER JOIN Publishers AS P On B.publisherId = P.id;";
        var allBooks = db.Query<Book, Publisher, Book>(sql, (book, publisher) =>
        {
             book.publisher = publisher;
             return book;
        });
        return allBooks;
    }
}

How to get the remaining data for a Book (ie Author and Tag data with the many-to-many relationships as seen in the schema)?

Answer 1

It wouldn't be very efficient if you go querying book by book so my recommendation is to query first all Author (s) including their Book.id then query all Tag (s) including their Book.id and then use LINQ to match by Book.id over the results of those queries as follows:

 public IEnumerable<Book> GetAllBooks() { using (var db = new SqliteConnection(connectionString)) { var authorsSql = "SELECT A.*, B.id AS BookId FROM Books as B " + "INNER JOIN Book_Author AS B2A On B.id = B2A.bookId " + "INNER JOIN Author A On B2A.authorId = A.id;"; var allAuthorsWithBookId = db.Query<dynamic>(authorsSql).AsList(); var tagsSql = "SELECT T.*, B.id AS BookId FROM Books as B " + "INNER JOIN Book_Tag AS B2T On B.id = B2T.bookId " + "INNER JOIN Tags T On B2T.tagId = T.id;"; var allTagsWithBookId = db.Query<dynamic>(tagsSql).AsList(); var sql = "SELECT * FROM Books as B " + "INNER JOIN Publishers AS P On B.publisherId = P.id;"; var allBooks = db.Query<Book, Publisher, Book>(sql, (book, publisher) => { book.publisher = publisher; book.Authors = allAuthorsWithBookId.Where(row => row.BookId == book.id).Select(row => new Author{ id = row.id, name = row.name }).AsList(); book.Tags = allTagsWithBookId.Where(row => row.BookId == book.id).Select(row => new Tag{ id = row.id, name = row.name }).AsList(); return book; }); return allBooks; } }

You'll need to fix some typos and structural code (like a parenthesis at the end of the using ) and some missing properties in classes Author , Tag , etc. but it should be OK after that.