如何建立mgo查询？

Question

I'm doing a tiny project(practice) with revel and mgo, but I got something wrong with search function when I build the queries. The code seems like this:

conditions := make(bson.M, 0)
conditions["status"] = bson.M{"$ne": "delete"}

if item, ok := paramsPost["title"]; ok {
    if item[0] != "" {
        conditions["title"] = bson.RegEx{Pattern: item[0]}
    }
}
if item, ok := paramsPost["from_date"]; ok {
    if item[0] != "" {
        conditions["publishdate"] = bson.M{}
        fromDate, _ := time.Parse("2006-01-02", item[0])
        conditions["publishdate"]["$gte"] = fromDate.Unix()
    }
}

if item, ok := paramsPost["to_date"]; ok {
    if _, ok := conditions["publishdate"]; !ok {
        conditions["publishdate"] = bson.M{}
    }
    if item[0] != "" {
        toDate, _ := time.Parse("2006-01-02", item[0])
        conditions["publishdate"]["$lte"] = toDate.Unix()
    }
}

And I got some error info:

invalid operation: conditions["publishdate"]["$gte"] (index of type interface {})

I know I make something wrong, but I don't know why, and how to resolve. Anybody can help me? Thanks

Answer 1

bson.M is a map[string]interface{} (http://godoc.org/labix.org/v2/mgo/bson#M)

So, in

conditions["publishdate"]["$gte"] = fromDate.Unix()

You need to do a type assertion from interface{} to bson.M when looking up publishdate in the map.

Instead, you could refactor the code to something like

publishdate:= bson.M{}
// ... your logic goes here
conditions["publishdate"] = publishDate

to save on unnecessary map lookups and type assertions.