显示React迭代中的最后一个子级 [英] Display Last Child in React Iteration
问题描述
我正在尝试实施一个消息应用程序,暂时没有ActionCable。当我映射所有消息时,我只想显示该用户的最后一条消息:我只显示最后一条,而不是显示某个用户的所有消息。我不知道该如何实现。我以为可以将 key
设置为 user_id
而不是 id
。
I'm trying to implement a message app, without ActionCable for now. When I map over all messages, I want to show only the last message from that user: instead of showing all messages, from a user, I only show the last. I do not know how to achieve that. I thought I could set the key
to the user_id
instead of the id
.
_messagesRender: function(){
var messages = this.props.message.map( function(m, i){
return(
<li key={m.user_id}>{m.body}</li>
)
});
return(
<div>{messages.length === 0 ? "No messages" : messages}</div>
)
},
该组件位于索引页面上,我正在尝试创建类似消息的应用。有React的指针吗? Rails 5用作后端。
This component is on the index page and I'm trying to create a message-like app. Any pointers with React? Rails 5 is used as back end.
警告:flattenChildren(...):遇到两个具有相同键的孩子,
。$ 2
。子密钥必须唯一。当两个孩子共享一个密钥时,只会使用第一个孩子。
Warning: flattenChildren(...): Encountered two children with the same key,
.$2
. Child keys must be unique; when two children share a key, only the first child will be used.
后端:
@messages = Message.where(to: current_user.id)
消息模型::id,:to,:user_id,:body
user_id
是发送/创建消息的人。
user_id
is who sends/created the message.
推荐答案
您可以尝试以下操作:
Message.where(to: current_user.id)
.order(:created_at)
.group_by(&:user_id)
.map{|_, x| x.last}
这应该给出最后一条消息(由 created_at
)来自每个 user_id
。
This should give the last message (by created_at
) from each user_id
.
只要您使用的是Postgresql,就可以尝试区别于
方法:
As long as you are using Postgresql, you can try distinct on
approach:
Message.where(to: current_user.id)
.order(user_id: :asc, created_at: :desc)
.select('distinct on (user_id) *')
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