Geospatial queries basics in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a geospatial query in MongoDB?

A geospatial query in MongoDB is a way to find documents based on their location data, such as points on a map, using coordinates like latitude and longitude.

beginner

Which MongoDB index type is used for 2D geospatial queries?

The '2d' index type is used for flat, two-dimensional geospatial queries on legacy coordinate pairs.

beginner

What does the $near operator do in a geospatial query?

The $near operator finds documents with locations closest to a specified point, sorting results by distance from that point.

intermediate

Explain the difference between '2d' and '2dsphere' indexes in MongoDB.

'2d' indexes work with flat coordinate systems for simple x,y points, while '2dsphere' indexes support spherical geometry for Earth-like curved surfaces and GeoJSON data.

intermediate

What is GeoJSON and why is it important in MongoDB geospatial queries?

GeoJSON is a format for encoding geographic data structures like points, lines, and polygons. MongoDB uses GeoJSON with '2dsphere' indexes to perform accurate spherical geospatial queries.

Which MongoDB operator finds documents near a specific point?

A $geoWithin

B $near

C $geoIntersects

D $match

$near finds documents closest to a given point, sorted by distance.

What type of index should you create for spherical Earth data in MongoDB?

A 2d

B text

C 2dsphere

D hashed

2dsphere indexes support spherical geometry and GeoJSON data.

Which format does MongoDB use to store complex geospatial shapes like polygons?

A GeoJSON

B XML

C CSV

D JSON

GeoJSON is the standard format for geographic shapes in MongoDB.

What does the $geoWithin operator do?

A Finds documents near a point

B Sorts documents by distance

C Finds documents intersecting a line

D Finds documents within a specified shape

$geoWithin finds documents located inside a given geometry like a polygon.

Which index type is best for legacy flat coordinate pairs?

A 2d

B 2dsphere

C text

D hashed

2d indexes are designed for flat, two-dimensional coordinate pairs.

Describe how MongoDB uses geospatial indexes to perform location-based queries.

Think about how maps can be flat or round like the Earth.

Explain the role of GeoJSON in MongoDB geospatial queries and why it matters.

GeoJSON is like a language for describing shapes on maps.

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