Tag: appengine

Here’s a demonstration of how to query the datastore in Go.

In this example we filter on PropertyOne, requiring it to be equal to true. You can also set other inequalities such as greater than ( > ). Kind is the kind of the entities to query, and PropertyTwodemonstrates ordering by descending order. CustomStruct is the struct that was used to create the entity. Remember to put your entity processing code just before the last brace ( } ).

//Search the datastore for entities
q := datastore.NewQuery(kind).Filter("PropertyOne =", true).Order("-PropertyTwo")
//Loop through each returned entity.
for t := q.Run(c); ; {
    //This represents the entity currently being processed.
    var x CustomStruct
    key, err := t.Next(&x)
    if err == datastore.Done {
        //This "error" means that we're done going through entities
        //Since we're done, break out of this loop.
        break
    }
    if err != nil {
        //Some other error happened. Report error and panic.
        c.Infof("Error in querying datastore: %v", err)
        panic(err)
    }
    //DO SOMETHING HERE WITH ENTITY x
}//end for going through q.Run

Here’s a short code example demonstrating how to delete an entity in Go. C is an appengine.Contextreference, kind is the entity kind, and entity_id_int represents the integer ID of the entity.

key := datastore.NewKey(c, kind, "", entity_id_int, nil)
datastore.Delete(c, key)

Here is a code snippet demonstrating how to retrieve a datastore entity by a known kind and integer ID. This code also retrieves the entity ID from a HTTP parameter.

R represents a http.Request reference. The entity ID to retrieve is accessed from the user’s submitted form under the name entity_id. Kind represents a string containing the name of the kind to retrieve from the datastore. The retrieved entity is stored into the variable entity, which is the struct CustomStruct.

c := appengine.NewContext(r)
//Retrieve the entity ID from the submitted form. Convert to an int64
entity_id := r.FormValue("entity_id")
entity_id_int, err := strconv.ParseInt(entity_id, 10, 64) 
if err != nil {
    fmt.Fprint(w, "Unable to parse key")
    return;
}
//We manufacture a datastore key based on the Kind and the 
//entity ID (passed to us via the HTTP request parameter.
key := datastore.NewKey(c, kind, "", entity_id_int, nil)
//Load the Entity this key represents.
//We have to state the variable first, then conduct the Get operation 
//so the datastore understands the struct representing the entity.
var entity CustomStruct
datastore.Get(c, key, &entity)

In short, this code pulls out the Entity ID from the HTTP request, converts it into an integer, manufactures the entity’s key using the kind and entity id, and then retrieves the corresponding entity. The code then translates the retrieved entity into the defined CustomStruct variable entity.