FindOne document without a particular element [duplicate]

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You can try this:

   "class": {
      "$exists": false
}).then(text => res.json(text))

.css-rjswxq{font-size:unset;}$addToSet only ensures that there are no duplicate items added to the set and does not affect existing duplicate elements. .css-rjswxq{font-size:unset;}$addToSet does not guarantee a particular ordering of elements in the modified set.,To add each element of the value separately, use the .css-rjswxq{font-size:unset;}$each modifier with $addToSet. See $each Modifier for details.,If the value is an array, .css-rjswxq{font-size:unset;}$addToSet appends the whole array as a single element.

.leafygreen-ui-iokrs{color:inherit;font-size:13px;font-family:'Source Code Pro',Menlo,monospace;line-height:24px;}.leafygreen-ui-1v41da1{border-spacing:0;width:100%;}.leafygreen-ui-7razhx{border-spacing:0;vertical-align:top;padding:0 16px;}{ $addToSet: { <field1>: <value1>, ... } }
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Finding duplicate values in your database can be difficult, especially if you have millions of documents to look at. MongoDB's aggregation pipeline makes finding duplicate documents easier by allowing you to customize how documents are grouped together and filtered. In other words, MongoDB lets you select fields and group together documents based on your selection in order to find duplicate documents.,Need to find duplicate documents in your MongoDB database? This article will show you how to find duplicate documents in your existing database using MongoDB's aggregation pipeline.,One you've run mongoimport in the terminal, you should see that 559 documents have been successfully imported into your database. Now, let's log into our MongoDB shell and look at our documents.

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As we know that set does not contain any duplicate element we will be using the set here.,Find the two numbers with odd occurrences in an unsorted array,Find position of an element in a sorted array of infinite numbers


References to other documents,Queries are Not Promises,Anywhere a callback is passed to a query in Mongoose, the callback follows the pattern callback(error, results). What results is depends on the operation: For findOne() it is a potentially-null single document, find() a list of documents, count() the number of documents, update() the number of documents affected, etc. The API docs for Models provide more detail on what is passed to the callbacks.

When executing a query with a callback function, you specify your query as a JSON document. The JSON document's syntax is the same as the MongoDB shell.

const Person = mongoose.model('Person', yourSchema);

// find each person with a last name matching 'Ghost', selecting the `name` and `occupation` fields
   'name.last': 'Ghost'
}, 'name occupation', function(err, person) {
   if (err) return handleError(err);
   // Prints "Space Ghost is a talk show host".
   console.log('%s %s is a %s.',,,
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Find a duplicate in an array,Before to see the solutions, let’s talk a bit about the problem. We get an array of n + 1 element with integers between 1 and n.,There is another solution which also considers the given array as a sort of linked list (again, this is possible because of the constraint that each integer is between 1 and n).

Given an array of n + 1 integers between 1 and n, find one of the duplicates.If there are multiple possible answers,
   return one of the duplicates.If there is no duplicate,
      return -1. Example: Input: [1, 2, 2, 3] Output: 2 Input: [3, 4, 1, 4, 1] Output: 4 or 1
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For me, learning even the simplest of things can take time. Mostly because I have trouble searching for it. As I’ve had no formal training in programming, I’m ignorant of a lot of technical terms that would make searching online so much more easier.,If there are no matches in your database, you can proceed with saving or updating your code however you want to.,So once again I was stuck trying to figure out how to make sure I didn’t save duplicates in my database. This is useful for document titles or objects that you want to be unique. Like a username. If you don’t put up any red tape in your code, then you’ll end up with a lot of duplicates which can cause trouble later.

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