Command syntax

Description

Provide a default value in case of non-existence errors. The default command evaluates its first argument (the value it’s chained to). If that argument returns null or a non-existence error is thrown in evaluation, then default returns its second argument. The second argument is usually a default value, but it can be a function that returns a value.

Example: Retrieve the titles and authors of the table posts. In the case where the author field is missing or null, we want to retrieve the string Anonymous.

r.table("posts").map(function (post) {
    return {
        title: post("title"),
        author: post("author").default("Anonymous")
    }
}).run(conn, callback);

We can rewrite the previous query with r.branch too.

r.table("posts").map(function (post) {
    return r.branch(
        post.hasFields("author"),
        {
            title: post("title"),
            author: post("author")
        },
        {
            title: post("title"),
            author: "Anonymous" 
        }
    )
}).run(conn, callback);

Example: The default command can also be used to filter documents. Retrieve all our users who are not grown-ups or whose age is unknown (i.e., the field age is missing or equals null).

r.table("users").filter(function (user) {
    return user("age").lt(18).default(true)
}).run(conn, callback);

One more way to write the previous query is to set the age to be -1 when the field is missing.

r.table("users").filter(function (user) {
    return user("age").default(-1).lt(18)
}).run(conn, callback);

This can be accomplished with hasFields rather than default.

r.table("users").filter(function (user) {
    return user.hasFields("age").not().or(user("age").lt(18))
}).run(conn, callback);

The body of every filter is wrapped in an implicit .default(false). You can overwrite the value false with the default option.

r.table("users").filter(function (user) {
    return user("age").lt(18)
}, {default: true} ).run(conn, callback);

Example: The function form of default receives the error message as its argument.

r.table("posts").map(function (post) {
    return {
        title: post("title"),
        author: post("author").default(function (err) {
            return err;
        })
    }
}).run(conn, callback);

This particular example simply returns the error message, so it isn’t very useful. But it would be possible to change the default value based on the specific error message thrown.