投影阵列中项目阶段操作员的行为 [英] Behaviour of the project stage operator in projecting Arrays

问题描述

我的问题与 this ，但不相似。

My question is closely related to this, but not similar.

我的收藏夹中有一个示例文档：

I have a sample document in my collection:

db.t.insert({"a":1,"b":2});

我的意图是设计一个名为 combined array 的>，其值分别为 a 和 b （[1,2]）。

My intent is to project a field named combined of type array with the values of both a and b together.([1,2]).

我只是尝试将 $ project 阶段：

db.t.aggregate([
{$project:{"combined":[]}}
])

MongoDB 引发错误：不允许对象表达式中的字段类型为数组。

但是当我使用 $ cond 运算符来投影一个数组时，

But when i use a $cond operator to project an array, the field gets projected.

db.t.aggregate([
{$project:{"combined":{$cond:[{$eq:[1,1]},["$a","$b"],"$a"]}}}
])

我得到o / p： { combined：[ $ a， $ b ]} 。

I get the o/p: {"combined" : [ "$a", "$b" ] }.

如果您注意到输出，则 a 和 b 就像是文字而不是字段路径一样。

If you notice the output, the value of a and b are treated as if they were literals and not a field paths.

有人可以向我解释这种现象吗？，当我使条件失败时，

Can anyone please explain to me this behavior?, When i make the condition to fail,

db.t.aggregate([
{$project:{"combined":{$cond:[{$eq:[1,2]},["$a","$b"],"$a"]}}}
])

我得到了预期的输出，其中 $ a 被视为字段路径，因为 $

I get the expected output where $a is treated as a field path, since $a is not enclosed as an array element.

推荐答案

我以前也遇到过这个问题，并且这很烦人，但实际上按文字 [ $ a， $ b] 所记录的那样工作；关于不允许的字段类型的第一个错误是...不清楚为什么会抱怨。但是，您必须遵循文档中扩展的 $ project 阶段的语法说明。我会在这里尝试这样做。从 $ project ，

I've run into this before too and it's annoying, but it's actually working as documented for literal ["$a", "$b"]; the first error about disallowed field type is...not as clear why it complains. You have to follow the description of the grammar of the $project stage spread out in the documentation, however. I'll try to do that here. Starting at $project,

$ project阶段具有以下原型形式：

The $project stage has the following prototype form:

{ $project: { <specifications> } }

其规格可以是以下之一：

and specifications can be one of the following:

1. <field> : <1 or true or 0 or false>
2. <field> : <expression>

什么是表达式？来自聚合表达式，

表达式可以包括字段路径和系统变量，文字，表达式对象和运算符。

Expressions can include field paths and system variables, literals, expression objects, and operator expressions.

这些东西分别是什么？字段路径/系统变量应该很熟悉：它是一个字符串文字，以$或$$为前缀。表达式对象的格式为

What are each of those things? A field path/system variable should be familiar: it's a string literal prefixed with $ or $$. An expression object has the form

{ <field1>: <expression1>, ... }

而运算符表达式具有以下形式之一

while an operator expression has one of the forms

{ <operator>: [ <argument1>, <argument2> ... ] }

{ <operator>: <argument> }

获得< operator> 。

什么是< argument> ？文档尚不清楚，但是根据我的经验，我认为它是任何表达式，但要遵循给定运算符的语法规则（检查运算符表达式 cond：...

What's an <argument>? The documentation isn't clear on it, but from my experience I think it's any expression, subject to the syntax rules of the given operator (examine the operator expression "cond" : ... in the question).

数组只能作为参数列表和文字的容器。文字是文字-不会为字段路径或系统变量评估其内容，这就是为什么 $ cond 中的数组文字参数最终以值 [ $ a， $ b] 。对参数数组中的表达式进行求值。

Arrays fit in only as containers for argument lists and as literals. Literals are literals - their content is not evaluated for field paths or system variables, which is why the array literal argument in the $cond ends up with the value [ "$a", "$b" ]. The expressions in the argument array are evaluated.

关于数组是不允许的值类型的第一个错误对我来说有点奇怪，因为数组文字是有效的表达式，因此根据文档，它可以是对象表达式中的值。在将其解析为对象表达式的一部分时，我也没有任何歧义。看起来这只是他们简化解析的规则？您可以使用 $ literal 来躲避它，以放置一个恒定的数组值：

The first error about Array being a disallowed value type is a bit odd to me, since an array literal is a valid expression, so according to the documentation it can be a value in an object expression. I don't see any ambiguity in parsing it as part of an object expression, either. It looks like it's just a rule they made to make the parsing easier? You can "dodge" it using $literal to put in a constant array value:

db.collection.project([{ "$project" : { "combined" : { "$literal" : [1, 2] } } }])

我希望这有助于解释为什么事情会这样进行。第一次尝试执行 [ $ a， $ b] 之类的操作时，我感到很惊讶，但它没有按预期工作。至少有一个功能可以将字段路径打包到一个数组中。当 $ group 也对有序值对进行查找时，我也找到了它的用途。

I hope this helps explain why things work this way. I was surprised the first time I tried to do something like [ "$a", "$b" ] and it didn't work as I expected. It'd be nice if there were a feature to pack field paths into an array, at least. I've found uses for it when $grouping on ordered pairs of values, as well.

有一个JIRA票证， SERVER-8141 ，请求 $ array 运算符可帮助处理此类情况。

There's a JIRA ticket, SERVER-8141, requesting an $array operator to help with cases like this.

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