Why Do My Google App Engine Datastore Queries Fail to Retrieve Newly Added Entities in Tests?

Google App Engine Datastore - Persistent Queries

In the context of testing, accessing data in Google App Engine's Datastore through queries can be challenging. Despite seemingly successful functionality in the application, tests often fail to retrieve expected data.

One potential reason for this discrepancy lies in the delayed consistency of datastore queries. Unlike ancestor queries, most queries require some time to reflect newly added entities. The line "Applying all pending transactions and saving the datastore" indicates that the testing framework is applying queued transactions after the entity has been added and queried.

As a result, queries run immediately after entity creation may not capture these pending changes. To address this, there are several options:

• Introduce a Delay: Introduce a short delay (e.g., 100ms) between the data creation and subsequent query to allow for consistency.
• Use Strongly Consistent Datastore: Enable the StronglyConsistentDatastore option during test configuration to force immediate data consistency.

Here is a modified version of your test code using the delayed approach:

type Entity struct {
    Value string
}

func TestEntityQuery(t *testing.T) {
    c, err := aetest.NewContext(nil)
    if err != nil {
        t.Fatal(err)
    }
    defer c.Close()

    key := datastore.NewIncompleteKey(c, "Entity", nil)
    key, err = datastore.Put(c, key, &Entity{Value: "test"})
    if err != nil {
        t.Fatal(err)
    }

    // Delay to allow for consistency
    time.Sleep(100 * time.Millisecond)

    q := datastore.NewQuery("Entity").Filter("Value =", "test")
    var entities []Entity
    keys, err := q.GetAll(c, &entities)
    if err != nil {
        t.Fatal(err)
    }
    if len(keys) == 0 {
        t.Error("No keys found in query")
    }
    if len(entities) == 0 {
        t.Error("No entities found in query")
    }
}

By incorporating these measures, tests can accurately reflect the behavior of datastore queries and ensure that duplicate entities are not inadvertently created.

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