Dates in MongoDB


My colleague came to me last Friday and asked me to help him out on one of the issues he faced with MongoDB’s query. He found that the JSON data received via the API when our BI tool inserted the data into MongoDB, it saved the date and datetime values in string format.

The JSON specification does not specify a format for exchanging dates which is why there are so many different ways to do it. The best format is the ISO date format, it is a well known and widely used format and can be handled across many different languages, making it very well suited for interoperability.

It shall look like below:

2012-04-23T18:25:43.511Z

When I took over the JSON data and found that most of the date and time are in normal string format (eg: 2012-04-23 18:25:43.511), MongoDB inserted the data into the database as string type.

To understand dates in MongoDB, I found this article is pretty good in explaining to them, here is the link. Instantly, it hit me! Dealing with dates in MongoDB or any databases is not an easy job at all, moreover, this time I am dealing with MongoDB where it has subdocument or array in a subdocument structure.

I can use the $dateFromString which converts a date/time string to a date object. For more detail about this function, you can find it out at this link.

This aggregation operator worked like magic to me when I am dealing with the date and time in the string until I reached a point where the date is in an array inside a subdocument. I faced a roadblock and I am not able to use the same method to do the conversion. It seems quite true that I have to use $unwind in my query. It deconstructs an array field from the input documents to output a document for each element. Each output document is the input document with the value of the array field replaced by the element.

I need a solution that I can use $unwind to flatten the array from the collection and convert each date and time into a date object. I am still looking for a solution, if you happened to solve something similar like mine, please give me a helping hand and link me to the solutions. Thank you!

Maybe useful: https://stackoverflow.com/questions/38299186/query-that-combines-project-unwind-group

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