Attribute pattern for variable fields in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents with variable field names matching a pattern

Given a MongoDB collection products where some documents have fields like attr_color, attr_size, and others have different attr_* fields, which query returns all documents that have at least one field starting with attr_?

A { $expr: { $gt: [ { $size: { $objectToArray: '$$ROOT' } }, 0 ] } }

B { $or: [ { 'attr_*': { $exists: true } } ] }

C { $expr: { $gt: [ { $size: { $filter: { input: { $objectToArray: '$$ROOT' }, cond: { $regexMatch: { input: '$$this.k', regex: '^attr_' } } } } }, 0 ] } }

D { attr: { $regex: '^attr_' } }

💡 Hint

Use $objectToArray to convert fields to key-value pairs and filter keys by regex.

Option C converts the document fields to an array, filters keys starting with 'attr_', and checks if any exist. This matches documents with at least one 'attr_' field.

📝 Syntax

intermediate

Identify the syntax error in a MongoDB query filtering variable fields

Which of the following MongoDB queries has a syntax error when trying to find documents with fields starting with data_?

A db.collection.find({ $expr: { $gt: [ { $size: { $filter: { input: { $objectToArray: '$$ROOT' }, cond: { $regexMatch: { input: '$$this.k', regex: '^data_' } } } } }, 0 ] } })

B db.collection.find({ $expr: { $gt: [ { $size: { $filter: { input: { $objectToArray: '$ROOT' }, cond: { $regexMatch: { input: '$$this.k', regex: '^data_' } } } } }, 0 ] } })

C )} } ] 0 ,} } } } } '_atad^' :xeger ,'k.siht$$' :tupni { :hctaMxeger$ { :dnoc ,} 'TOOR$$' :yarrAoTtcejbo$ { :tupni { :retlif$ { :ezis$ { [ :tg$ { :rpxe$ {(dnif.noitcelloc.bd

D b.collection.find({ $expr: { $gt: [ { $size: { $filter: { input: { $objectToArray: '$$ROOT' }, cond: { $regexMatch: { input: '$$this.k', regex: '^data_' } } } } }, 0 ] } })

💡 Hint

Check the variable referencing the root document inside $objectToArray.

Option B uses '$ROOT' instead of '$$ROOT' which is invalid syntax for referencing the root document variable in aggregation expressions.

🧠 Conceptual

advanced

Understanding attribute pattern matching in MongoDB documents

Why is it necessary to use $objectToArray and $filter to find documents with fields matching a pattern like attr_* in MongoDB?

A Because $objectToArray converts values to keys, allowing regex matching on values instead of keys.

B Because MongoDB automatically indexes all fields starting with 'attr_', so $objectToArray optimizes the query.

C Because $filter can only be used on arrays, and documents are arrays by default in MongoDB.

D Because MongoDB does not support querying field names directly with regex, so fields must be converted to array elements to filter keys.

💡 Hint

Think about how MongoDB stores documents and how queries work on field names.

MongoDB queries cannot directly apply regex on field names. Converting the document to an array of key-value pairs allows filtering keys by regex.

❓ optimization

advanced

Optimizing queries for variable attribute fields in MongoDB

Which approach is the most efficient to find documents with any field starting with meta_ in a large MongoDB collection?

A Store all variable fields inside a subdocument named 'meta' and query with dot notation like 'meta.field'.

B Create a wildcard index on all fields and query with a regex on the field name.

C Use $objectToArray and $filter with $regexMatch inside an aggregation pipeline to filter documents.

D Use $where with JavaScript to iterate over fields and check if any field name starts with 'meta_'.

💡 Hint

Consider indexing and query performance on large collections.

Storing variable fields inside a subdocument allows indexing and efficient querying using dot notation, which is faster than scanning all fields or using JavaScript.

🔧 Debug

expert

Debugging a MongoDB query for variable attribute fields

A developer wrote this query to find documents with fields starting with 'info_': db.collection.find({ $expr: { $gt: [ { $size: { $filter: { input: { $objectToArray: '$$ROOT' }, cond: { $regexMatch: { input: '$$this.k', regex: '/^info_/' } } } } }, 0 ] } }) Why does this query return no results even though matching documents exist?

A The regex pattern is incorrectly wrapped in slashes, causing it to be treated as a string literal.

B The variable '$$this.k' is invalid and should be '$$this.key'.

C The $filter input should be '$ROOT' instead of '$$ROOT'.

D The $size operator cannot be used inside $expr.

💡 Hint

Check how regex patterns are specified in MongoDB aggregation expressions.

In MongoDB aggregation expressions, regex patterns should be strings without slashes. Using '/^info_/' treats it as a string literal, so no keys match.