查找缺少任意字段的 CouchDB 文档 [英] Find CouchDB docs missing an arbitrary field

问题描述

我需要一个 CouchDB 视图，我可以在其中取回所有没有任意字段的文档.如果您事先知道文档可能没有哪些字段，这很容易做到.例如，这可以让你发送 view/my_view/?key="foo" 轻松检索没有 "foo" 字段的文档:

I need a CouchDB view where I can get back all the documents that don't have an arbitrary field. This is easy to do if you know in advance what fields a document might not have. For example, this lets you send view/my_view/?key="foo" to easily retrieve docs without the "foo" field:

function (doc) {
  var fields = [ "foo", "bar", "etc" ];

  for (var idx in fields) {
    if (!doc.hasOwnProperty(fields[idx])) {
      emit(fields[idx], 1);
    }
  }
}

但是，您只能询问视图中设置的三个字段；像 view/my_view/?key="baz" 这样的东西不会给你任何东西，即使你有很多文档缺少该字段.我需要一个视图——我不需要提前指定可能缺少的字段.有什么想法吗?

However, you're limited to asking about the three fields set in the view; something like view/my_view/?key="baz" won't get you anything, even if you have many docs missing that field. I need a view where it will--where I don't need to specify possible missing fields in advance. Any thoughts?

推荐答案

这种技术被称为泰式按摩.当(且仅当)视图以文档 ID 为键时，使用它在视图中不有效地查找文档.

This technique is called the Thai massage. Use it to efficiently find documents not in a view if (and only if) the view is keyed on the document id.

function(doc) {
    // _view/fields map, showing all fields of all docs
    // In principle you could emit e.g. "foo.bar.baz"
    // for nested objects. Obviously I do not.
    for (var field in doc)
        emit(field, doc._id);
}

function(keys, vals, is_rerun) {
    // _view/fields reduce; could also be the string "_count"
    return re ? sum(vals) : vals.length;
}

要查找没有该字段的文档，

To find documents not having that field,

1. GET/db/_all_docs 并记住所有 ID
2. GET/db/_design/ex/_view/fields?reduce=false&key="some_field"
3. 比较 _all_docs 中的 id 与查询中的 id.

1. GET /db/_all_docs and remember all the ids
2. GET /db/_design/ex/_view/fields?reduce=false&key="some_field"
3. Compare the ids from _all_docs vs the ids from the query.

_all_docs 中但不在视图中的 id 是缺少该字段的那些.

The ids in _all_docs but not in the view are those missing that field.

将 id 保存在内存中听起来很糟糕，但您不必这样做！您可以使用合并排序策略，同时迭代两个查询.您从 has 列表的第一个 id(来自视图)和 full 列表的第一个 id(来自 _all_docs)开始.

It sounds bad to keep the ids in memory, but you don't have to! You can use a merge sort strategy, iterating through both queries simultaneously. You start with the first id of the has list (from the view) and the first id of the full list (from _all_docs).

1. 如果满 <有，缺少字段，用下一个完整元素重做
2. 如果full = has，它有字段，用下一个full元素重做
3. 如果 full > has，用下一个 has 元素重做

1. If full < has, it is missing the field, redo with the next full element
2. If full = has, it has the field, redo with the next full element
3. If full > has, redo with the next has element

根据您的语言，这可能很困难.但在 Javascript 或其他事件驱动的编程框架中非常容易.

Depending on your language, that might be difficult. But it is pretty easy in Javascript, for example, or other event-driven programming frameworks.

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