在两个集合上进行MongoDB地理搜索

Question

I'm trying to figure out a way to geo query two collections:

1. Person {name, age, groupid}
2. Groups (id, name, geoLocation[x,y])

I need to find all near groups with persons with age > 18

Any idea how to do it without having to query the groups for each found person?

Answer 1

You have no joins in MongoDB so you need to find another way around this.

The next plausable method I would think of would be to store an array of age ranges with the group record:

{
    _id: {},
    name: {},
    geoLocation: {},
    age_groups: {
        'gt18': 1
    },
}

And then I would just query on that age_groups.gt18 field and pull out all records. This does mean, of course, you will require something to keep this field upto date. There are a couple of methods:

• MR (Map Reduce) This would actually entail outputting to a summary table first so I don't recommend this really.
• Event based Pre-aggregation. When a user joins or leaves a group you get their age and update the aggregated field accodingly.

I would personally go for the event based pre-aggregated method of doing things