查找管道内的 ObjectId 匹配无法正常工作

Question

The issue part is { $match: { contentId: "$$content_id" }} inside pipeline. It returns a empty array. but when I change it like below, it works fine. This is a stage object of my aggregation pipeline. What is the issue with the first? Simple match is designed not to work in lookup pipeline when let variable is an objectId? Thank you for your time.

const contentReactionLookUp = (user_id) =>  { 
    return {
        $lookup: {
            from: "contentreactions",
            let: { content_id: "$_id" },
            pipeline: [
                { $match:
                     { $expr:


                                     { $eq: [ "$contentId", "$$content_id" ] },


                     }
                },
                //{ $match: { contentId: "$$content_id" }}, it doenst' work! 
            ],
            as: "userReaction"
        },
    }
}

Answer 1

https://docs.mongodb.com/manual/meta/aggregation-quick-reference/#aggregation-expressions I found my solution here. mongo aggregation simple match doesn't work in case its passed value is field path.