Geospatial queries basics in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents within a circle

Given a MongoDB collection places with documents containing a location field as GeoJSON points, what will be the output of this query?

db.places.find({ location: { $geoWithin: { $centerSphere: [[-73.97, 40.77], 0.0001] } } })

The query finds documents within a circle centered at longitude -73.97, latitude 40.77 with radius 0.0001 radians.

A Documents with locations roughly within 637 meters of the point (-73.97, 40.77)

B Documents with locations exactly at (-73.97, 40.77) only

C Documents with locations within 0.01 degrees of the point (-73.97, 40.77)

D Documents with locations anywhere in the collection

💡 Hint

Remember that $centerSphere uses radians for radius, not degrees or meters.

The $centerSphere operator defines a circle on the Earth's surface using radians. Radius 0.0001 radians corresponds to about 637 meters (0.0001 * Earth's radius ≈ 6371 km). So the query returns documents whose location is within that circle.

🧠 Conceptual

intermediate

Understanding 2dsphere index usage

Which statement about MongoDB's 2dsphere index is correct?

A It is used only for legacy coordinate pairs, not GeoJSON.

B It automatically converts all coordinates to degrees before indexing.

C It supports queries on GeoJSON objects and calculates distances on a spherical surface.

D It only supports flat 2D plane calculations and cannot handle GeoJSON points.

💡 Hint

Think about how Earth’s curvature affects distance calculations.

The 2dsphere index supports GeoJSON objects and performs calculations assuming a spherical Earth, which is important for accurate geospatial queries.

📝 Syntax

advanced

Identify the syntax error in a geospatial query

Which option contains a syntax error in the MongoDB geospatial query?

MongoDB

db.places.find({ location: { $near: { $geometry: { type: "Point", coordinates: [-73.97, 40.77] }, $maxDistance: 1000 } } })

A db.places.find({ location: { $near: { $geometry: { type: "Point", coordinates: [-73.97, 40.77] }, maxDistance: 1000 } } })

B db.places.find({ location: { $near: { $geometry: { type: "Point", coordinates: [-73.97, 40.77] }, $maxDistance: 1000 } } })

C db.places.find({ location: { $near: { $geometry: { type: "Point", coordinates: [40.77, -73.97] }, $maxDistance: 1000 } } })

D db.places.find({ location: { $nearSphere: { $geometry: { type: "Point", coordinates: [-73.97, 40.77] }, $maxDistance: 1000 } } })

💡 Hint

Check the spelling and syntax of the distance option inside $near.

In MongoDB geospatial queries, the distance option must be $maxDistance with a dollar sign. Option A uses maxDistance without the dollar sign, causing a syntax error.

❓ optimization

advanced

Optimizing geospatial queries with indexes

You have a collection restaurants with a location field storing GeoJSON points. Which index will optimize queries that find restaurants near a given point?

A { location: "2d" }

B { location: "2dsphere" }

C { location: "hashed" }

D { location: 1 }

💡 Hint

Consider the type of geospatial data stored and the query type.

The 2dsphere index supports GeoJSON points and spherical calculations, which are needed for near queries on Earth coordinates. The 2d index is for flat coordinate pairs and does not support GeoJSON.

🔧 Debug

expert

Why does this geospatial query return no results?

You run this query on a MongoDB collection parks with a location field indexed as 2dsphere:

db.parks.find({ location: { $geoWithin: { $box: [[-74, 40], [-73, 41]] } } })

But it returns no documents, even though you know some parks are inside that box. What is the most likely reason?

A The $geoWithin operator requires $centerSphere, not $box.

B The coordinates in $box are reversed; longitude and latitude are swapped.

C The collection does not have any documents with location fields.

D The $box operator only works with 2d indexes, not 2dsphere indexes.

💡 Hint

Check compatibility of operators with index types.

The $box operator works only with legacy 2d indexes and flat coordinate pairs. It is not supported with 2dsphere indexes, which require GeoJSON geometry operators like $geometry or $centerSphere.

Practice

1. What does the $near operator do in MongoDB geospatial queries?

easy

A. Finds documents with locations closest to a specified point

B. Deletes documents within a certain distance

C. Updates documents with new coordinates

D. Sorts documents alphabetically by location name

Solution

1. Step 1: Understand the purpose of $near

   The $near operator is used to find documents that are geographically close to a given point.

2. Step 2: Compare options with $near functionality

   Only Finds documents with locations closest to a specified point correctly describes finding documents near a location. Other options describe unrelated actions.

3. Final Answer:

   Finds documents with locations closest to a specified point -> Option A

4. Quick Check:

   $near = Finds nearby points [OK]

Hint: Remember: $near finds closest points to a location [OK]

Common Mistakes:

• Thinking $near deletes or updates documents
• Confusing $near with sorting operators
• Assuming $near works without a geospatial index

2. Which of the following is the correct syntax to find documents near a point using $near in MongoDB?

easy

A. { location: { $near: { $geometry: { type: 'Point', coordinates: [-70, 40] } } } }

B. { location: { $near: { $geometry: { type: 'Point', coordinates: [40, -70] } } } }

C. { location: { $near: { $geometry: { type: 'Point', coordinates: [70, 40] } } } }

D. { location: { $near: { $geometry: { type: 'Point', coordinates: [-40, 70] } } } }

Solution

1. Step 1: Recall coordinate order in MongoDB

   Coordinates must be in [longitude, latitude] order, so longitude comes first.

2. Step 2: Check each option's coordinate order

   { location: { $near: { $geometry: { type: 'Point', coordinates: [-70, 40] } } } } uses [-70, 40], which is [longitude, latitude]. Others have latitude first or wrong signs.

3. Final Answer:

   { location: { $near: { $geometry: { type: 'Point', coordinates: [-70, 40] } } } } -> Option A

4. Quick Check:

   Coordinates = [longitude, latitude] [OK]

Hint: Coordinates always [longitude, latitude] in MongoDB [OK]

Common Mistakes:

• Swapping latitude and longitude order
• Using positive latitude as longitude
• Forgetting to use $geometry inside $near

3. Given a collection with a 2dsphere index on location, what will this query return?

{ location: { $near: { $geometry: { type: 'Point', coordinates: [-73.97, 40.77] }, $maxDistance: 500 } } }

medium

A. All documents regardless of location

B. Documents with locations within 500 meters of [-73.97, 40.77]

C. Documents with locations more than 500 meters away

D. Documents with locations exactly at [-73.97, 40.77]

Solution

1. Step 1: Understand $near with $maxDistance

   The query finds documents near the point within the maximum distance specified (500 meters).

2. Step 2: Interpret the query result

   It returns documents whose location is within 500 meters of the given coordinates, not exactly at or beyond that distance.

3. Final Answer:

   Documents with locations within 500 meters of [-73.97, 40.77] -> Option B

4. Quick Check:

   $near + $maxDistance = nearby points within distance [OK]

Hint: Use $maxDistance to limit how far results can be [OK]

Common Mistakes:

• Thinking $maxDistance means exact match
• Assuming it returns points beyond maxDistance
• Ignoring the need for a 2dsphere index

4. You run this query but get an error:

{ location: { $near: { $geometry: { type: 'Point', coordinates: [40.77, -73.97] } } } }

What is the most likely cause?

medium

A. Coordinates are in wrong order; should be [longitude, latitude]

B. Missing $maxDistance parameter

C. The location field is not indexed with 2dsphere

D. The type should be 'Polygon' instead of 'Point'

Solution

1. Step 1: Requirements for $near with $geometry

   The $near operator with $geometry requires a 2dsphere index on the location field.

2. Step 2: Identify the most likely error cause

   Without a 2dsphere index, MongoDB throws an error for geospatial queries like $near. Coordinate order affects results but does not cause an error.

3. Final Answer:

   The location field is not indexed with 2dsphere -> Option C

4. Quick Check:

   $near requires 2dsphere index [OK]

Hint: Always put longitude before latitude in coordinates [OK]

Common Mistakes:

• Swapping latitude and longitude
• Assuming $maxDistance is required
• Confusing geometry types

5. You want to find all restaurants within 1 km of a user's location stored as { type: 'Point', coordinates: [-122.42, 37.77] }. Which query correctly finds these restaurants assuming a 2dsphere index on location?

hard

A. { location: { $near: { $geometry: { type: 'Point', coordinates: [-122.42, 37.77] }, $minDistance: 1000 } } }

B. { location: { $geoWithin: { $centerSphere: [[-122.42, 37.77], 1] } } }

C. { location: { $nearSphere: { $geometry: { type: 'Point', coordinates: [37.77, -122.42] }, $maxDistance: 1000 } } }

D. { location: { $near: { $geometry: { type: 'Point', coordinates: [-122.42, 37.77] }, $maxDistance: 1000 } } }

Solution

1. Step 1: Identify correct coordinate order and operator

   Coordinates must be [longitude, latitude], so [-122.42, 37.77] is correct. The $near operator with $maxDistance finds points within distance.

2. Step 2: Evaluate each option

   { location: { $near: { $geometry: { type: 'Point', coordinates: [-122.42, 37.77] }, $maxDistance: 1000 } } } uses correct coordinates and $near with $maxDistance 1000 meters (1 km). { location: { $geoWithin: { $centerSphere: [[-122.42, 37.77], 1] } } } uses $geoWithin with radius in radians, not meters. { location: { $nearSphere: { $geometry: { type: 'Point', coordinates: [37.77, -122.42] }, $maxDistance: 1000 } } } swaps coordinates. { location: { $near: { $geometry: { type: 'Point', coordinates: [-122.42, 37.77] }, $minDistance: 1000 } } } uses $minDistance which finds points farther than 1 km, not within.

3. Final Answer:

   { location: { $near: { $geometry: { type: 'Point', coordinates: [-122.42, 37.77] }, $maxDistance: 1000 } } } -> Option D

4. Quick Check:

   $near + $maxDistance + correct coords = find nearby points [OK]

Hint: Use $near with $maxDistance and correct coords for nearby search [OK]

Common Mistakes:

• Swapping latitude and longitude
• Using $minDistance instead of $maxDistance
• Confusing meters with radians in $geoWithin