Dynamically query mongodb with golang

Question

I'm trying to query my mongodb database using golang (and the mgo library) with only one function, and the method I am currently using is:

er = c.Find(sel(items)).Sort("-createdAt").All(&result)

Where items is a map and the key is the name of the field I am searching inthe db, and the value is what I want to search by.

and sel() is:

func sel(query map[string]string) bson.M {
result := make(bson.M, len(query))
result[ ] = "$in"
for k, v := range query {
    result[k] = v
}
return result

currently it will return all of the results where at least one of the fields matches the input map. (So a logical OR) however I would like it to return the logical AND of these fields.

Does anyone have suggestions on how to modify the existing code or a new way of efficiently querying the database?

Thank you

Can you elaborate the logical AND behavior you desire? Matching all the fields, not any of them? — vahdet
– vahdet, Commented Jun 27, 2018 at 18:48
I am hoping to get a result where each value in the map matches the corresponding field in the database. So I would like it to be something like bson.M{"$and" : all of the k,v pairs} — bagreeni
– bagreeni, Commented Jun 27, 2018 at 18:52

icza · Accepted Answer · 2018-06-27 20:18:58Z

4

I don't know what this line is supposed to mean:

result[ ] = "$in"

As it is a compile-time error.

But the elements of the query document (the conditions) are in logical AND connection by default, so this is all it takes:

func sel(query map[string]string) bson.M {
    result := make(bson.M, len(query))
    for k, v := range query {
        result[k] = v
    }
    return result
}

If this gives you all the documents in the collection, then that means all the key-value pairs match all the documents. Experiment with simple filters to see that it works.

Also note that the mgo package also accepts a wide range of maps and structs, not just bson.M. Documentation of Collection.Find() has this to say about the allowed types:

The document may be a map or a struct value capable of being marshalled with bson. The map may be a generic one using interface{} for its key and/or values, such as bson.M, or it may be a properly typed map. Providing nil as the document is equivalent to providing an empty document such as bson.M{}.

So you can use your map which is of type map[string]string without converting it:

err = c.Find(items).Sort("-createdAt").All(&result)

edited Jun 27, 2018 at 20:18

answered Jun 27, 2018 at 19:19

icza

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4 Comments

bagreeni Over a year ago

I forgot to remove that line before copying! Thank you. Also we just found a bug in the code from a colleague of mine that was causing our "query" problem that turns out wasn't even a db issue... Gotta love human error

icza Over a year ago

@bagreeni Please see my edit. You don't even need this sel() conversion: you can just use your original map[string]string map without conversion!

bagreeni Over a year ago

Thank you! This is extremely helpful to know

icza Over a year ago

@bagreeni I provided exact description (with reference) about what can be used as the query.

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