My goal is to search for all keys within a collection that partially match a specific string.
So far, I've only been able to check if a key exists, but it has to be an exact match. Here's the code I've used:
db.collection.find({ "fkClientID": { $exists:1 }})What I really need is to find all keys that begin with fk instead.
If you want to extract field names from your MongoDB collection using mapReduce, here's how you can achieve that:
To retrieve only the field names at the root level:
db.collection.mapReduce(function () {
Object.keys(this).forEach(function(key) {
if (key.match(/^fk/)) emit(key, null);
});
}, function() {}, { out: { inline: 1 }});
This will provide an output similar to this:
{
"results": [{
"_id": "fkKey1",
"value": null
}, {
"_id": "fkKey2",
"value": null
}],
"timeMillis": W,
"counts": {
"input": X,
"emit": Y,
"reduce": Z,
"output": 2
},
"ok" : 1
}
To get both field names and their values:
db.collection.mapReduce(function () {
var obj = this;
Object.keys(this).forEach(function(key) {
if (key.match(/^fk/)) emit(key, obj[key]);
});
}, function(key, values) {
return { values: values };
}, { out: { inline: 1 }});
The results would look like this:
{
"results": [{
"_id": "fkKey1",
"value": {
"values": [1, 4, 6]
}
}, {
"_id": "fkKey2",
"value": {
"values": ["foo", "bar"]
}
}],
"timeMillis": W,
"counts": {
"input": X,
"emit": Y,
"reduce": Z,
"output": 2
},
"ok" : 1
}
For extracting field names in subdocuments:
In order to fetch field names within subdocuments, consider storing JavaScript functions on the server:
db.system.js.save({ _id: "hasChildren", value: function(obj) {
return typeof obj === "object";
}});
db.system.js.save({ _id: "getFields", value: function(doc) {
Object.keys(doc).forEach(function(key) {
if (key.match(/^fk/)) emit(key, null);
if (hasChildren(doc[key])) getFields(doc[key])
});
}});
Don't forget to run db.loadServerScripts() after adding these scripts.
If you are working with the latest version of MongoDB 3.4.4, then utilizing $objectToArray in an aggregate statement alongside $redact is incredibly efficient. This method allows for native operators to be used which results in a very fast processing speed compared to other methods available.
db[collname].aggregate([
{ "$redact": {
"$cond": {
"if": {
"$gt": [
{ "$size": { "$filter": {
"input": { "$objectToArray": "$$ROOT" },
"as": "doc",
"cond": {
"$eq": [ { "$substr": [ "$$doc.k", 0, 2 ] }, "fk" ]
}
}}},
0
]
},
"then": "$$KEEP",
"else": "$$PRUNE"
}
}}
])
The functionality of $objectToArray which transforms an "object" into key-value pairs in an array format provides great insight into document structure. With the use of $$ROOT, representing the current document object, we can easily extract and examine these key-value pairs.
For obtaining a unique list of matching keys, the optimized approach in MongoDB 3.4.4 would involve:
db[collname].aggregate([
{ "$redact": {
"$cond": {
"if": {
"$gt": [
{ "$size": { "$filter": {
"input": { "$objectToArray": "$$ROOT" },
"as": "doc",
"cond": {
"$eq": [ { "$substr": [ "$$doc.k", 0, 2 ] }, "fk" ]
}
}}},
0
]
},
"then": "$$KEEP",
"else": "$$PRUNE"
}
}},
{ "$project": {
"j": {
"$filter": {
"input": { "$objectToArray": "$$ROOT" },
"as": "doc",
"cond": {
"$eq": [ { "$substr": [ "$$doc.k", 0, 2 ] }, "fk" ]
}
}
}
}},
{ "$unwind": "$j" },
{ "$group": { "_id": "$j.k" }}
])
Alternatively, if certain conditions are met where the key is always present and there is only one occurrence, the aggregation process can be simplified by using $group:
db[colname].aggregate([
{ "$group": {
"_id": {
"$arrayElemAt": [
{ "$map": {
"input": { "$filter": {
"input": { "$objectToArray": "$$ROOT" },
"as": "doc",
"cond": {
"$eq": [ { "$substr": [ "$$doc.k", 0, 2 ] }, "fk" ]
}
}},
"as": "el",
"in": "$$el.k"
}},
0
]
}
}}
])
In earlier versions of MongoDB, utilizing the $where syntax was prevalent for evaluating JavaScript expressions. While not the most efficient method due to JavaScript evaluation overhead, it allowed for dynamic analysis of keys within documents:
db[collname].find(function() { return Object.keys(this).some( k => /^fk/.test(k) ) })
This expression can also be structured using standard query syntax as demonstrated below:
db[collname].find({ "$where": "return Object.keys(this).some( k => /^fk/.test(k) )" })
Ultimately, the choice between different approaches depends on the specific requirements of your data model and processing needs.
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