如果我有Order子句，则Google App Engine数据存储区不返回任何行

Question

I have a 'kind' in datastore like so:

type CompanyDS struct {
    Name string
}

If i query it with the 'order' clause below, it returns no rows (but doesn't give any error):

var companiesDS []CompanyDS
datastore.NewQuery("Company").Order("Name").GetAll(c, &companiesDS)

However if i remove the 'order("Name")' section it returns all the rows just fine.

Answer 1

Since without Order() you can query all entities, that means they do exist with name "Company" and property "Name".

Indices for single properties are automatically created, so you don't need to specify explicit index for them.

But if you can't list them using a single property ordering like Order("Name"), that means that your existing entities are not indexed with the Name property. Note that every single entity may be indexed differently. When you save (put) an entity into the Datastore, you have the ability to specify which properties are to be indexed and which are not.

You can confirm this on the Google Cloud Platform Datastore console: execute the query

select * from Company

Then click on any of the results (its ID), then you will see the details of that entity, listing which property is indexed and which is not.

Fix:

You may edit the entities on the console: click on the "Name" property, and before saving, check the "Index this property". This will re-save this entity, making its Name indexed, and thus it will show up in the next query (ordered by Name).

You don't need to do this manually for all entities. Use your Go query code (without Order()), query all entities, then re-save all without modification, and so the Name will get indexed as a result of this (because your CompanyDS does not turn off indexing for the Name property). Make sure your struct contains all properties, else you would lose them when re-saving.

Note: You should ensure that the code that saves Company entities saves them with Name indexed.

In Go for example a struct tag with value ",noindex" will disable indexing for a single property like in this example:

type CompanyDS struct {
    Name string `datastore:",noindex"`
}