Bienvenue dans le débordement de pile. C'est une bonne question, vous avez posté. S'il vous plaît laissez-moi prendre le privilège de vous aider de la façon dont je peux.
Il s'agit d'une commande d'agrégation qui peut être exécutée dans le shell mongo. Veuillez trouver l'explication en ligne.
db.collection.aggregate([
//match all those records which involve Wood.
{$match:{$or:[{"to":"wood"},{"from":"wood"}]}},
// sort all the messages by descending order
{$sort:{time:-1}},
{
// Now we need to group messages together, based on the to and from field.
// We generate a key - "last_message_between", with the value being a concatenation
// result of the values in to and from field.
// Now, Messages from Wood to billy and Billy to wood should be treated under a single group right.
// To achieve that, we do a small trick to make the result contain the name coming last
// alphabetically, first. So our key for all the Messages from Wood to Billy and Billy to Wood would be
// "Wood and Billy".
// And then we just display the first document that appears in the group, that would be the
// latest Message.
$group:{"_id":{
"last_message_between":{
$cond:[
{
$gt:[
{$substr:["$to",0,1]},
{$substr:["$from",0,1]}]
},
{$concat:["$to"," and ","$from"]},
{$concat:["$from"," and ","$to"]}
]
}
},"message":{$first:"$$ROOT"}
}
}
])
Vous pouvez exécuter ce qui suit sur la mangouste.
Collection.aggregate(
{$match:{$or:[{"to":"wood"},{"from":"wood"}]}},
{$sort:{time:-1}},
{
$group:{"_id":{
"last_message_between":{
$cond:[
{
$gt:[
{$substr:["$to",0,1]},
{$substr:["$from",0,1]}]
},
{$concat:["$to"," and ","$from"]},
{$concat:["$from"," and ","$to"]}
]
}
},"message":{$first:"$$ROOT"}
}
},
function(err, res)
{
if (err) return handleError(err);
console.log(res);
}
)
