MongoDB has a $max operator that allows you to update the value of a field only if the specified value is greater than the current value of the field.
In other words, if the $max value is greater than the current value in the document, the $max value is used. Otherwise, the document’s value remains unchanged.
MongoDB has a $min operator that allows you to update the value of a field only if the specified value is less than the current value of the field.
In other words, if the $min value is less than the current value in the document, the $min value is used. Otherwise, the document’s value remains unchanged.
In MongoDB, you can use the $pop operator to remove the first or last element from an array.
Use $pop in conjunction with a method such as update() to update the specified document with the change.
Use -1 to remove the first element, and 1 to remove the last.
In MongoDB, you can use the $pullAll operator to remove all instances of the specified values from an existing array.
Use $pullAll in conjunction with a method such as update(), updateOne(), or updateMany() to update the specified document/s with the change.
In MongoDB, you can use the $pull operator to remove values from an array.
Use $pull in conjunction with a method such as update() to update the specified document with the change.
Sometimes when you query a collection in MongoDB, you might not be happy with the field names. By default, the field names are simply a reflection of the field names in the actual documents.
Perhaps the field names are inconsistent, or there’s a typo. Whatever the reason, you can use the $project aggregation pipeline stage to rename a field in your query results.
In some ways, this is comparable to using an alias in SQL, as it doesn’t rename the underlying fields, it simply renames them in the query results.
Continue readingIn MongoDB, the $addToSet operator adds a value to an array unless the value is already present in the array.
It’s similar to the $push operator, except that $push adds the value even if the value is already present.
In MongoDB, you can use the $push operator to append a value to an array.
You can use various modifiers to specify the value’s position in the array, the order of the elements in the array, append multiple values, etc.
Continue readingWhen you use methods such as find() or findOne() in MongoDB, by default you get the whole document returned. And if you use projections, you can return specific key/value pairs.
But what if you only want the value?
You can extract the value of a field by appending that field’s name to your query when using findOne().
If you’re familiar with SQL, you might know about the UNION clause, which concatenates the results of two queries into a single result set. In particular, UNION ALL includes duplicates.
In MongoDB, we can use the $unionWith aggregation pipeline stage to achieve the same effect that UNION ALL produces. The $unionWith stage performs a union of two collections – it combines pipeline results from two collections into a single result set. And it includes duplicates.