mongodb 0.2.8

use mongodb::common::{ReadMode, ReadPreference};
use serde_json::{Map, Value};
use std::collections::BTreeMap;
use std::str::FromStr;

use super::super::FromValueResult;

impl FromValueResult for ReadPreference {
    fn from_json(object: &Map<String, Value>) -> Result<ReadPreference, String> {
        let mode = val_or_err!(object.get("mode"),
                               Some(&Value::String(ref s)) => ReadMode::from_str(s).unwrap(),
                               "read preference must have a mode.");

        let tag_sets_array = val_or_err!(object.get("tag_sets"),
                                         Some(&Value::Array(ref arr)) => arr.clone(),
                                         "read preference must have tag sets");

        let mut tag_sets_objs = Vec::new();
        let mut tag_sets = Vec::new();

        for json in &tag_sets_array {
            match *json {
                Value::Object(ref obj) => tag_sets_objs.push(obj.clone()),
                _ => return Err(String::from("tags must be document objects.")),
            }
        }

        for obj in tag_sets_objs {
            let mut tags = BTreeMap::new();
            for (ref key, ref json) in obj {
                match *json {
                    Value::String(ref s) => {
                        tags.insert(key.to_owned(), s.to_owned());
                    }
                    _ => return Err("tags must be string => string maps.".to_owned()),
                }
            }
            tag_sets.push(tags);
        }

        Ok(ReadPreference::new(mode, Some(tag_sets)))
    }
}