go-generic-pgx

Playing around with Go 1.18's new Generics feature. (... requires go1.18+ to run )

Current supports generic Query execution and QueryRow execution.

Given these tables:

create table products(product_no integer, product_name text, price integer)
create table purchases(purchase_no integer, product_no integer, price_paid integer, customer_id text)

This works by building a generic querier object. This leverages the faciliator pattern to support that Golang does not have parameterized methods.

First, we can define the columns we are scanning with an ordered array of pgtype.Value interface values.

var ProductColumns = []pgtype.Value{&pgtype.Int4{}, &pgtype.Text{}, &pgtype.Int4{}}

With this, we can build a callback function that operates on this array to map to a specific struct.

func ProductMapper(a ...pgtype.Value) Product {
	p := Product{}

	num, _ := a[0].(*pgtype.Int4)
	p.Number = int(num.Int)

	str, _ := a[1].(*pgtype.Text)
	p.Name = str.String

	price, _ := a[2].(*pgtype.Int4)
	p.Price = int(price.Int)

	return p
}

Note that this is not a huge improvement in typing code, but one thing to note is that this is much easier to test than the current interface provided by database/sql package. I personally think this callback method is a lot easier to reason about as the callback function doesn't hide the nitty-gritty postgres details, e.g. could this column be null.

With the columns we are scanning defined, and a mapping callback to map the columns into a Go struct, we can now execute a query.

// instantiate a new querier
query := `select product_no, product_name, price from products where product_no = $1`
q := NewQuerier(conn, query, ProductColumns, ProductMapper)

// now we can use this querier to get our product directly back
product, err := q.QueryRow(context.Background(), 0)

// Prints {0 a 100}
fmt.Println(product)

// Now lets reuse that logic for our purchase query
query = `select purchase_no, product_no, price_paid, customer_id, purchased_at from purchases where purchase_no = $1`
q1 := NewQuerier(conn, query, PurchaseColumns, PurchaseMapper)

purchase, err := q1.QueryRow(context.Background(), 0)

// Prints {0 1 1000 alice 2022-01-18 07:34:28.796139 -0700 MST}
fmt.Println(purchase)