Reusing entity cause slice values to be appended

[yuichi1004]

I found that when we fetch slice values from cache, goon appends the values into existing slice. Instead of appending values, goon should replace the slice.

Test to reproduce:
yuichi1004@ab412e6

Test Result:

$ go test -v -run Migration  ./
=== RUN   TestMigration
INFO     2018-04-28 09:43:06,511 devappserver2.py:105] Skipping SDK update check.
WARNING  2018-04-28 09:43:06,511 devappserver2.py:121] DEFAULT_VERSION_HOSTNAME will not be set correctly with --port=0
WARNING  2018-04-28 09:43:06,566 simple_search_stub.py:1196] Could not read search indexes from /var/folders/5j/2bmf8d2d3xgb9lmttlptxvm8p49fjs/T/appengine.testapp.01099286/search_indexes
INFO     2018-04-28 09:43:06,568 api_server.py:308] Starting API server at: http://localhost:51010
INFO     2018-04-28 09:43:06,638 dispatcher.py:255] Starting module "default" running at: http://localhost:51011
INFO     2018-04-28 09:43:06,640 admin_server.py:146] Starting admin server at: http://localhost:51013
--- FAIL: TestMigration (4.48s)
        goon_test.go:994: MC-false > Expected 1 car! Got: 2
FAIL
exit status 1
FAIL    github.com/yuichi1004/goon      4.501s

Note that we have extra cars on the above test.

[xStrom]

Appending without reset should be correct because this is what the datastore package does.

See the following quotes from the docs:

func Get

The values of dst's unmatched struct fields are not modified, and matching slice-typed fields are not reset before appending to them. In particular, it is recommended to pass a pointer to a zero valued struct on each Get call.

func RunInTransaction

Since f may be called multiple times, f should usually be idempotent. datastore.Get is not idempotent when unmarshaling slice fields.

[yuichi1004]

i never know this. thanks for your clarification. let me close this since it is as expected.