Safely perform DB migrations with Go - mysql

Question

Let's say I have a web app that shows a list of posts. The post struct is:
type Post struct {
Id int64 `sql:",primary"`
Title string
Body string
}
It retrieves the posts with:
var posts []*Post
rows, err := db.QueryContext(ctx, "select * from posts;")
if err != nil {
return nil, oops.Wrapf(err, "could not get posts")
}
defer rows.Close()
for rows.Next() {
p := &Post{}
err := rows.Scan(
&p.Id,
&p.Title,
&p.Body,
)
if err != nil {
return nil, oops.Wrapf(err, "could not scan row")
}
posts = append(posts, p)
}
return posts, nil
All works fine. Now, I want to alter the table schema by adding a column:
ALTER TABLE posts ADD author varchar(62);
Suddenly, the requests to get posts result in:
sql: expected 4 destination arguments in Scan, not 3
which makes sense since the table now has 4 columns instead of the 3 stipulated by the retrieval logic.
I can then update the struct to be:
type Post struct {
Id int64 `sql:",primary"`
Title string
Body string
Author string
}
and the retrival logic to be:
for rows.Next() {
p := &Post{}
err := rows.Scan(
&p.Id,
&p.Title,
&p.Body,
&p.Author
)
if err != nil {
return nil, oops.Wrapf(err, "could not scan row")
}
posts = append(posts, p)
}
which solves this. However, this implies there is always a period of downtime between migration and logic update + deploy. How to avoid that downtime?
I have tried swapping the order of the above changes but this does not work, with that same request resulting in:
sql: expected 3 destination arguments in Scan, not 4
(which makes sense, since the table only has 3 columns at that point as opposed to 4);
and other requests resulting in:
Error 1054: Unknown column 'author' in 'field list'
(which makes sense, because at that point the posts table does not have an author column just yet)

Answer 1

You should be able to achieve your desired behavior by adapting the SQL Query to return the exact fields you want to populate.
SELECT Id , Title , Body FROM posts;
This way even if you add another column Author the query results only contain 3 values.