I noticed a new $eq operator released with MongoDB 3.0 and I don't understand the purpose of it. For instance these two queries are exactly the same:
db.users.find({age:21})and
db.users.find({age:{$eq:21}})Does anyone know why this was necessary?
Best Answer
The problem was that you'd have to handle equality differently from comparison when you had some kind of query builder, so it's
{ a : { $gt : 3 } }{ a : { $lt : 3 } }but
{ a : 3 }for equality, which looks completely different. The same applies for composition of $not, as JohnnyHK already pointed out. Also, comparing with $eq saves you from having to $-escape user provided strings. Therefore, people asked for alternatives that are syntactically closer and it was implemented. The Jira ticket contains a longer discussion which mentions all these points.
The clearer syntax of an $eq operator might also make sense in the aggregation framework to compare two fields, should such a feature be implemented.
Also, the feature has apparently been around since 2.5, was added to the documentation relatively late.
One specific application I can see for $eq is with cases like the $not operator which requires that its value is an operator-expression.
This allows you to construct a query like:
db.zips.find({state: {$not: {$eq: 'NY'}}})Before, the closest you could get to this semantically was:
db.zips.find({state: {$not: {$regex: /^NY$/}}})I realize there are other ways to represent the functionality of that query, but if you need to use the $not operator there for other reasons, this would now allow it.
In filter part of an aggregation query if you need to check if some field is equal to a value you can not use assign syntax:
db.sales.aggregate([{$project: {items: {$filter: {input: "$items",as: "item",cond: { $eq: [ "$$item.price", 100 ] }}}}}])