如何从mongodb获取唯一记录？

I have a collection named 'myplace'. It has the following fields: place_name, latitude, longitude, city, country.

I want all the cities that start with the letter "A". I tried the following:

type Place struct{
    City string `bson: "City"`
}

For retrieving the result from the db:

var city_name []Place
err = coll.Find(bson.M{"city": bson.M{"$regex":"^a", "$options":"si"}}).All(&city_name)

It's getting all the results. The problem is that some of the 'myplace' documents have the same city, so it's returning duplicate city names.

Let's say I have 5 myplaces, 3 with the city name "Baton Rouge" and remaining having "Trivandrum, Kochi". When I try to get the city starting with "B", it's returning "Baton Rouge" 3 times.

How can I ensure each city_name is unique?

Thanks in advance

You can use the Distinct method, in the shell it will look like :

db.foo.distinct( "city",  { "city" : { "$regex" : /^A/i    } }  );

In Go:

 var result []string 

 err = c.Find( bson.M{"city": bson.M{"$regex":"^a","$options":"si"}}  ).Distinct("city", &result) 

 if err != nil {
        log.Fatal(err) 
 }

 fmt.Println( result )

You can use Distinct:

err = coll.Find(bson.M{"city": bson.M{"$regex":"^a", "$options":"si"}}).Distinct("city", &city_name)

..this assumes that city_name is actually a slice.