MongoDB query to retrieve distinct documents by date - mongodb

Question

I have documents in the database with a dateTime value like so:
{
"_id" : ObjectId("5a66fa22d29dbd0001521023"),
"exportSuccessful" : true,
"month" : 0,
"week" : 4,
"weekDay" : "Mon",
"dateTime" : ISODate("2018-01-22T09:02:26.525Z"),
"__v" : 0
}
I'd like to:
query the database for a given date and have it return the document that contains the dateTime if the date matches (I don't care about the time). This is mainly to test before inserting a document that there isn't already one for this date. In the above example, if my given date is 2018-01-22 I'd like the document to be returned.
retrieve all documents with a distinct date from the database (again, I don't care about the time portion). If there are two documents with the same date (but different times), just return the first one.
From what I understand Mongo's ISODate type does not allow me to store only a date, it will always have to be a dateTime value. And on my side, I don't have control over what goes in the database.

Answer 1

Try range query with start date time from start of the day to end date time to end of the day. So basically create dates a day apart.
Something like
var start = moment().utc().startOf('day');
var end = moment().utc().endOf('day');
db.collection.find({
dateTime: {
$gte: start,
$lte: end
}
})
Get all distinct dates documents:
db.collection.aggregate(
{"$group":{
"_id":{
"$dateToString":{"format":"%Y-%m-%d","date":"$dateTime"}
},
"first":{
"$first":"$$ROOT"
}
}}])