Add a ToSql method to BaseQuery to expose query args

The BaseQuery.String method returns the prepared SQL but does not return
the query arguments, making it hard to test queries without mocking the
database. By adding a BaseQuery.ToSql method that returns the args as
well as an error returned while compiling, anyone can easily test
queries built by a helper function and verify the sql and args are
correct.

query.go

@@ -284,6 +284,13 @@ func (q *BaseQuery) String() string {
 	return sql
 }
 
+// ToSql returns the SQL generated by the query, the query arguments, and
+// any error returned during the compile process.
+func (q *BaseQuery) ToSql() (string, []interface{}, error) {
+	_, builder := q.compile()
+	return builder.ToSql()
+}
+
 // ColumnOrder represents a column name with its order.
 type ColumnOrder interface {
 	// ToSql returns the SQL representation of the column with its order.

query_test.go

@@ -82,6 +82,16 @@ func (s *QuerySuite) TestString() {
 	s.Equal("SELECT __model.foo FROM model __model", s.q.String())
 }
 
+func (s *QuerySuite) TestToSql() {
+	s.q.Select(f("foo"))
+	s.q.Where(Eq(f("foo"), 5))
+	s.q.Where(Eq(f("bar"), "baz"))
+	sql, args, err := s.q.ToSql()
+	s.Equal("SELECT __model.foo FROM model __model WHERE __model.foo = $1 AND __model.bar = $2", sql)
+	s.Equal([]interface{}{5, "baz"}, args)
+	s.Equal(err, nil)
+}
+
 func (s *QuerySuite) TestAddRelation() {
 	s.Nil(s.q.AddRelation(RelSchema, "rel", OneToOne, nil))
 	s.Equal("SELECT __model.id, __model.name, __model.email, __model.age, __rel_rel.id, __rel_rel.model_id, __rel_rel.foo FROM model __model LEFT JOIN rel __rel_rel ON (__rel_rel.model_id = __model.id)", s.q.String())