how to use null parameter in mongodb? - mongodb

Question

I inputed a parameter, and using mongodb.
My query is
db.collections.find({"tags":parameter});
I want to run query, when parameter is "" (null or "").
It will be
db.collections.find({"tags":""});
It is returned empty value.
How can I use input parameter null or "" in mongoDB?
EDIT
I'm sorry. I'm beginner in here, so sorry.
I want to get all the values returned when I type null
For example, it looks like this my collections,
collections
{
"_id": 0,
"tags": ["happy", "sad", "nice", "bad"]
},
{
"_id": 1,
"tags": ["bad", "gloomy"]
}
I want the same result as below.
> Db.collections.find ({"tags": ""})
{
"_id": 0,
"tags": ["happy", "sad", "nice", "bad"]
},
{
"_id": 1,
"tags": ["bad", "gloomy"]
}
// Return all collections.
> Db.collections.find ({"tags": "happy"})
{
"_id": 0,
"tags": ["happy", "sad", "nice", "bad"]
}
// Return matching collections.
but, db.collections.find ({"tags": ""}) brings out the results of that result is empty.
How can I print out all the results when I enter a null value?

Answer 1

Since a null value can be represented in several ways, depending on the language that wrote to the database in the first place, you need to use a combinations of things. Your query will need to look something like
db.collection.find({$or:[{"tags":{"$type":"null"}}, {"tags": {"$exists":false}}, {"tags":""}]})
Since BSON has a concept of Null, we have type check to see if the field exists but simply has no value. In addition to this, the field could not exist at all, so this must be explicitly checked for. Finally, depending on the language and the way the field was serialized, an empty string is a possibility.
Note that
{"tags":null}
and
{"tags":{"$type":"null"}}
are essentially the same thing.
Here is a quick example
> db.test.insert({"abc":null})
WriteResult({ "nInserted" : 1 })
> db.test.find()
{ "_id" : ObjectId("56670b3072f096ee05a72063"), "abc" : null }
> db.test.find({"abc":{$type:10}})
{ "_id" : ObjectId("56670b3072f096ee05a72063"), "abc" : null }
> db.test.find({"abc":{$type:"null"}})
{ "_id" : ObjectId("56670b3072f096ee05a72063"), "abc" : null }
> db.test.find({"abc":null})
{ "_id" : ObjectId("56670b3072f096ee05a72063"), "abc" : null }
db.test.find({$or:[{"tags":{"$type":"null"}}, {"tags": {"$exists":false}}, {"tags":""}]})
{ "_id" : ObjectId("56670b3072f096ee05a72063"), "abc" : null }
As you can see they all work, although the last query is the most thorough way of testing.
EDIT OP CHANGED QUESTION
You cannot find all values when you type null. That is a value and potential state for a field. You need to do an implicit $and here to get what you want.
db.collection.find({tags:{$exists:true}, tags:{$in:["happy","sad"]}})
How do you actually assemble this in code? Well, that depends on your language, but here is some pseudo code.
def getTags(myTags):
if (tags is None):
db.collection.find({ tags: { "$exists": true } })
else:
db.collection.find({ tags: { "$exists": true }, tags: {"$in": myTags } })
You can also get crafty by using an explicit $and
def getTags(myTags):
query = [{ tags: { "$exists": true } }]
if (tags is Not None):
query.add({tags: {"$in": myTags } })
db.collection.find({ "$and": query })
I hope this answers your question more thoroughly.

Answer 2

The accepted answer isn't suitable for the question(at least nowadays).
In MongoDB >= 3.6.
you can use the $expr operator as follow :
assuming that your parameter name is tagsParam and its value may be an Array or null
db.collections.find({
$or: [
{ $expr: !tagsParam || !tagsParam.length },
{ tags: { $in: paramter } }
]
});
In this case you will get the desired result if the parameter value was ["happy"], null or even an empty array [].