[CAP] CQL Expand two levels deep

I’ve already made a post about expanding compositions or associations (see here), but this time I had to extend two levels deep.

My entity structure looked something to this:

entity Header: cuid, managed {
    Items           : Composition of many Header.Items
                          on Items.purgeRun = $self;
}

entity Header.Items : cuid, managed {
    file            : Association to Files @mandatory @assert.target;
    header: Association to Header;
}

entity Files : cuid, managed {          
    content         : LargeBinary @Core.MediaType: mediaType  @Core.ContentDisposition.Filename: fileName  @Core.ContentDisposition.Type: 'inline';
    mediaType       : String      @Core.IsMediaType: true;
    fileName        : String      @mandatory;
    size            : Integer;
}

It is possible to query all data in a single HTTP call using $expand twice.

### Header + Items + File
GET {{server}}/odata/v4/service/MainEntity/{{ID}}
?$expand=Items($expand=file)
Authorization: Basic {{username}} {{password}}

However, if you want to fetch the same data using CQL, the following query is required:

        const data = await SELECT(Header, ID)
            .columns(h => {
                h('*')
                h.Items(i => {
                    i('*')
                    i.file(f => f('*'))
                })
            })

This syntax is really hard to remember…

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