default

ReQL command: default

Command syntax

value.default(default_value | function) → any
sequence.default(default_value | function) → any

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 nil 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 nil, we want to retrieve the string Anonymous.

r.table("posts").map{ |post|
    {
        :title => post[:title],
        :author => post[:author].default("Anonymous")
    }
}.run(conn)

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

r.table("posts").map{ |post|
    r.branch(
        post.has_fields("author"),
        {
            :title => post[:title],
            :author => post[:author]
        },
        {
            :title => post[:title],
            :author => "Anonymous" 
        }
    )
}.run(conn)

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 nil).

r.table("users").filter{ |user|
    (user[:age] < 18).default(true)
}.run(conn)

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{ |user|
    user[:age].default(-1) < 18
}.run(conn)

This can be accomplished with has_fields rather than default.

r.table("users").filter{ |user|
    user.has_fields("age").not() | (user[:age] < 18)
}.run(conn)

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(:default => true) {|user|
    (user[:age] < 18)
}.run(conn)

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

r.table("posts").map{ |post|
    {
        :title => post["title"],
        :author => post["author"].default{ |err| err }
    }
}.run(conn)

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.

Get more help

Couldn't find what you were looking for?

© RethinkDB contributors
Licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License.
https://rethinkdb.com/api/ruby/default/

在线笔记
App下载
App下载

扫描二维码

下载编程狮App

公众号
微信公众号

编程狮公众号

意见反馈
返回顶部