$eq Query Operato

[Manuele Cappelleri]

Hello all,

can someone explain why this query does not throw an error?

{ $match: { 'key': {'$eq': { '$eq': 'value' } } }}

IMHO, this should lead to an error as it cannot be evaluated.

If I am not wrong, could we summarize a general rule stating that:

predicate to $eq cannot be an operator itself or an object containing keys that start with $

Thank you all in advance!

[Nishant Bhardwaj]

Hello Manuelle,

Few points to note here:

• By design, the $eq comparison query operator matches a field against a Value, Document value or an Array value.
• Field names must not start with a dollar sign (i.e. $), I recommend reviewing Naming Restrictions in the MongoDB manual.
• The inner most '$eq' will be treated as a field of a sub-document, which cannot exist as per the naming convention stated above.

Thus the query does not match any documents and returns no result.

Regards,

Nishant

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[Manuele Cappelleri]

Thank you Nishant,

the naming restriction was the main reason I wrote the post.

My point is: given that naming restriction disallow field names starting with $,

the above query SHOULD fail and throw an error as it is impossible to evaluate.

Allowing this query to run, imho, implicitly says that there MIGHT be a result, which is obviously wrong as per your reply....

Don't you agree?