pymongo教程(2)聚合操作

db.things.insert({"x": 1, "tags": ["dog", "cat"]})
db.things.insert({"x": 2, "tags": ["cat"]})
db.things.insert({"x": 2, "tags": ["mouse", "cat", "dog"]})
db.things.insert({"x": 3, "tags": []})

from bson.son import SON
db.things.aggregate([
    {"$unwind": "$tags"},
    {"$group": {"_id": "$tags", "count": {"$sum": 1}}},
    {"$sort": SON([("count", -1), ("_id", -1)])}
])
{'ok': 1.0, 'result': [{'count': 3, '_id': 'cat'}, {'count': 2, '_id': 'dog'}, {'count': 1, '_id': 'mouse'}]}

from bson.code import Code
mapper = Code("""
    function () {
      this.tags.forEach(function(z) {
        emit(z, 1);
      });
    }
""")
reducer = Code("""
    function (key, values) {
      var total = 0;
      for (var i = 0; i 
<p>map和reduce都是一个javascript的函数； map_reduce 方法会将统计结果保存到一个临时的数据集合中。</p>
<h2>Group</h2>
<p>group 操作与SQL的 GROUP BY 相似，同时比 Map/Reduce 要简单。</p>
<pre class="brush:php;toolbar:false">reducer = Code("""
    function(obj, prev){
      prev.count++;
    }
""")
results = db.things.group(key={"x":1}, condition={}, initial={"count": 0}, reduce=reducer)
for doc in results:
    print(doc)
{'x': 1.0, 'count': 1.0}
{'x': 2.0, 'count': 2.0}
{'x': 3.0, 'count': 1.0}