Retrieve relation one to many into JSON sql pure, Golang, Performance

Question

Suppose that I've the following structures that it's the mapped tables.

type Publisher struct{
   ID int       `db:"id"`
   Name  string `db:"name"`
   Books []*Book  
}

type Book struct {
   ID int       `db:"id"`
   Name string  `db:"name"`
   PublisherID   `db:"publisher_id"` 
}

So, What if I wanna retrieve all the Publisher with all related Books I would like to get a JSON like this:

[ //Publisher 1 
  {
    "id" : "10001", 
    "name":"Publisher1",
    "books" : [
       { "id":321,"name": "Book1"}, 
       { "id":333,"name": "Book2"}
    ]
  },
  //Publisher 2
  {
    "id" : "10002", 
    "name":"Slytherin Publisher",
    "books" : [
       { "id":4021,"name": "Harry Potter and the Chamber of Secrets"}, 
       { "id":433,"name": "Harry Potter and the Order of the Phoenix"}
    ]
  },
]

So I've the following structure that I use to retrieve all kind of query related with Publisher

type PublisherRepository struct{
   Connection *sql.DB
}
// GetEbooks return all the books related with a publisher
func (r *PublisherRepository) GetBooks(idPublisher int) []*Book {
    bs := make([]Book,0)
    sql := "SELECT * FROM books b WHERE b.publisher_id =$1 "
    row, err := r.Connection.Query(sql,idPublisher)
    if err != nil {
      //log
    }
    for rows.Next() {
      b := &Book{}
      rows.Scan(&b.ID, &b.Name, &b.PublisherID)
      bs := append(bs,b)
    }
    return bs
}
func (r *PublisherRepository) GetAllPublishers() []*Publisher {
    sql := "SELECT * FROM publishers"
    ps := make([]Publisher,0)
    rows, err := r.Connection.Query(sql)
    if err != nil { 
       // log 
    }
    for rows.Next() {
       p := &Publisher{}
       rows.Scan(&p.ID,&p.Name)
       // Is this the best way? 
       books := r.GetBooks(p.ID)
       p.Books  = books
    }
    return ps

}

So , here my questions

1. What is the best approach to retrieve all the publisher with the best performance, because a for inside a for is not the best solution, what if I've 200 publisher and in the average of each publisher has 100 books.

2. Is in GoLang idiomatic PublisherRepository or is there another way to create something to manage the transactions of an entity with pure sql?

Answer 1

1) Bad about this would be the sql request per iteration. So here a solution that does not make an extra request per Publisher:

func (r *PublisherRepository) GetAllPublishers() []*Publisher {
    sql := "SELECT * FROM publishers"
    ps := make(map[int]*Publisher)
    rows, err := connection.Query(sql)
    if err != nil { 
       // log 
    }
    for rows.Next() {
       p := &Publisher{}
       rows.Scan(&p.ID,&p.Name)
       ps[p.ID] = p
    }

    sql = "SELECT * FROM books"
    rows, err := connection.Query(sql)
    if err != nil {
      //log
    }
    for rows.Next() {
      b := &Book{}
      rows.Scan(&b.ID, &b.Name, &b.PublisherID)

      ps[b.PublisherID].Books = append(ps[b.PublisherID].Books, b)
    }

    // you might choose to keep the map as a return value, but otherwise:

    // preallocate memory for the slice
    publishers := make([]*Publisher, 0, len(ps))
    for _, p := range ps {
        publishers = append(publishers, p)
    }

    return publishers
}

2) Unless you create the PublisherRepository only once, this might be a bad idea creating and closing loads of connections. Depending also on your sql client implementation I would suggest (and also have seen it for many other go database clients) to have one connection for the entire server. Pooling is done internally by many of the sql clients, that is why you should check your sql client. If your sql client library does pooling internally use a global variable for the "connection" (it's not really one connection if pooling is done internally):

connection *sql.DB

func New () *PublisherRepository {
    repo := &PublisherRepository{}
    return repo.connect()
}

type PublisherRepository struct{
}

func (r *PublisherRepository) connect() *PublisherRepository {
    // open new connection if connection is nil 
    // or not open (if there is such a state)
    // you can also check "once.Do" if that suits your needs better
    if connection == nil {
        // ...
    }
    return r
}

So each time you create a new PublisherRepository, it will only check if connection already exists. If you use once.Do, go will only create the "connection" once and you are done with it.

If you have other structs that will use the connection as well, you need a global place for your connection variable or (even better) you write a little wrapper package for your sql client, that is in turn used in all your structs.

Answer 2

1. In your case, the simplest way would be to use json_agg in the query. Like here http://sqlfiddle.com/#!15/97c41/4 (sqlfiddle is slow so here is screenshot http://i.imgur.com/hxMPkUa.png) Not very Go friendly (you need to unmarshal query result data if you want to do something with the books) but all books in one query as you wanted without for loops.

2. As @TehSphinX said it is better to have single global db connection.

But before implementing strange queries I really suggest you to think: why do you need to return the full list of publishers and their books in one API query? I can't imagine the situation in web or mobile app where your idea might be a good decision. Usually, you just show users list of publishers then users chooses one and you show him the list of books by this publisher. This is "win-win" situation for you and your users - you can make simple queries and your users just get small sets of data that they actually need without paying for unnecessary traffic/wasting browser memory. As you said there can be 200 publishers with 100 books and I'm sure your users don't need 20000 books loaded in one request. Of course, if you are not trying to make your API more data theft friendly.

Even if you have something like a short preview-like list of books for each publisher you should think about pagination for publishers and/or denormalisation of books data for this case (add a column to publishers table with the short list of books in JSON format).