Putting many PropertyList's into Google App Engine datastore (in Go) and loading them again with Query.GetAll

I am putting entities (as a datastore.PropertyList) into the datastore like this:

// save one
var plist datastore.PropertyList = make(datastore.PropertyList, 3)
plist = append(plist, datastore.Property { "name", "Mat", false, false })
plist = append(plist, datastore.Property { "age", "29", false, false })
plist = append(plist, datastore.Property { "location", "London", false, false })
key := datastore.NewIncompleteKey(context, "Record", nil)
datastore.Put(context, key, &plist)

// save another one
var plist datastore.PropertyList = make(datastore.PropertyList, 3)
plist = append(plist, datastore.Property { "name", "Laurie", false, false })
plist = append(plist, datastore.Property { "age", "27", false, false })
plist = append(plist, datastore.Property { "location", "London", false, false })
key := datastore.NewIncompleteKey(context, "Record", nil)
datastore.Put(context, key, &plist)

That all works fine (although the code above is more like pseudo code for now). I am able to load them individually, and the datastore.PropertyList comes out with each field as its own datastore.Property.

However, when I try to retrieve many of them using a Query, it fails:

query := datastore.NewQuery("Record")

plists := make(datastore.PropertyList, 0, 10)
keys, err := query.GetAll(context, &plists)

I get the following error:

datastore: cannot load field "age" into a "datastore.Property": no such struct field

It seems that it doesn't complain about Name because that happens to be a valid property of datastore.Property, so how do I get it to load the items as intended, with each item in plists being a datastore.PropertyList instead of datastore.Property?

I changed the implementation to use the PropertyLoadSaver interface - you can see it working nicely in our new Active Record style wrapper for datastore: http://github.com/matryer/gae-records (see the record.go type's Load and Save methods)

> GetAll runs the query in the given context and returns all keys that match that query, as well as appending the values to dst. The dst must be a pointer to a slice of structs, struct pointers, or Maps. If q is a “keys-only” query, GetAll ignores dst and only returns the keys.

And according to the following post the go datastore module doesn't support PropertyList yet.

Use a pointer to a slice of datastore.Map instead.

Also note that you need to do call make([]T, n) in order to make a slice of T, not make(T, n)

Try writing your field names with a capital letter, for example, instead of age, write Age. This tells Go that your field is exported (it's similar to the concept of public variables, though it goes a bit further than that).

According to the Go datastore documentation (look at the Get method in the Functions section), a Get or GetAll method call, will return an ErrFieldMismatch "when a field is to be loaded into a different type than the one it was stored from, or when a field is missing or unexported in the destination struct. ErrFieldMismatch is only returned if dst is a struct pointer.". My best guess is that since you stored it as a PropertyList with unexported names, and since the datastore is flexible in its models, it will take the original values as unexported, due to their lowercase letters.

I am currently having the same error, but I managed to track it down to being due to some empty fields.

If this solves your problem, please let me know.