Add database picker master/slave when QueryRow is called

db.go

@@ -13,8 +13,9 @@ import (
 // forming a single master multiple slaves topology.
 // Reads and writes are automatically directed to the correct physical db.
 type DB struct {
-	pdbs  []*sql.DB // Physical databases
-	count uint64    // Monotonically incrementing counter on each query
+	pdbs       []*sql.DB // Physical databases
+	count      uint64    // Monotonically incrementing counter on each query
+	queryRowDB func(query string, args ...interface{}) *sql.DB
 }
 
 // Open concurrently opens each underlying physical db.
@@ -147,7 +148,12 @@ func (db *DB) QueryContext(ctx context.Context, query string, args ...interface{
 // Errors are deferred until Row's Scan method is called.
 // QueryRow uses a slave as the physical db.
 func (db *DB) QueryRow(query string, args ...interface{}) *sql.Row {
-	return db.Slave().QueryRow(query, args...)
+	d := db.Slave() // default to slave
+	if db.queryRowDB != nil {
+		// conditional function that decide what db to use
+		d = db.queryRowDB(query, args)
+	}
+	return d.QueryRow(query, args...)
 }
 
 // QueryRowContext executes a query that is expected to return at most one row.
@@ -206,3 +212,7 @@ func (db *DB) slave(n int) int {
 	}
 	return int(1 + (atomic.AddUint64(&db.count, 1) % uint64(n-1)))
 }
+
+func (db *DB) SetQueryRowDB(fn func(query string, args ...interface{}) *sql.DB) {
+	db.queryRowDB = fn
+}

db_test.go

@@ -1,6 +1,8 @@
 package nap
 
 import (
+	"database/sql"
+	"strings"
 	"testing"
 	"testing/quick"
 
@@ -59,3 +61,43 @@ func TestSlave(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+func TestQueryRow(t *testing.T) {
+	// https://www.sqlite.org/inmemorydb.html
+	db, err := Open("sqlite3", ":memory:;:memory:;:memory:")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+
+	if err = db.Ping(); err != nil {
+		t.Error(err)
+	}
+
+	master := false
+
+	db.SetQueryRowDB(func(query string, args ...interface{}) *sql.DB {
+		if len(query) > 12 && strings.ToLower(query)[0:12] == "insert into " {
+			master = true
+			return db.Master()
+		}
+		master = false
+		return db.Slave()
+	})
+
+	res := db.QueryRow("insert into t1(c1,c2) values(1,1);", nil)
+	if res == nil {
+		t.Errorf("func QueryRow has no results")
+	}
+	if !master {
+		t.Errorf("query row expected to use master database")
+	}
+
+	res = db.QueryRow("select * from t1", nil)
+	if res == nil {
+		t.Errorf("func QueryRow has no results")
+	}
+	if master {
+		t.Errorf("query row expected to use slave database")
+	}
+}