How to Retrieve Entities by an Array of IDs in Google App Engine Datastore using Golang?

Querying Datastore for Entities with an Array of IDs using Golang

In Google App Engine Datastore, there is no direct support for performing an "IN" query to retrieve entities based on an array of IDs. However, there are alternative approaches to achieve this functionality.

Query with Range (If IDs are Consecutive)

If the IDs form a continuous range, you can use the ">" (greater than) and "<=" (less than or equal to) operators to specify the range of IDs to query. For example:

<code class="go">ids := []int64{1, 2, 3, 4}
q := datastore.NewQuery("Category").Filter("Id>=", 1).Filter("Id<=", 4)</p>
<p><strong>Execute Multiple Queries</strong></p>
<p>For IDs that are not in a continuous range, you can execute a separate query for each ID in the array. This involves creating a new Query object for each ID and filtering by that ID.</p>
<pre class="brush:php;toolbar:false"><code class="go">for _, id := range ids {
    q := datastore.NewQuery("Category").Filter("Id =", id)
    // ... perform query and retrieve entities
}</code>

GetMulti Function

If the property you are filtering on is the entity key itself, you can use the datastore.GetMulti() function to retrieve a list of entities based on an array of their keys.

<code class="go">var keys []*datastore.Key

for _, id := range ids {
    keys = append(keys, datastore.NewKey(c, "Category", "", id, nil))
}

categories := make([]Category, len(keys))
err := datastore.GetMulti(c, keys, categories)
if err != nil {
    // Handle error
}</code>

Note:

When using the Filter() method to specify multiple filters, they will be joined with an AND operation. Therefore, your attempt to filter by multiple IDs using q.Filter("Id =", id) will likely result in no matching entities unless all the IDs exist in the same entity.

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