Fixing bug with only __typename in the selection set

[severussundar]

Why make this change?

• Closes Selection set without entity fields returns an error response for GraphQL query and mutation #1463
• Queries and Mutations with only _typename in the selection set fails at the database layer because there are no columns to choose in the database query.

     {
        book_by_pk(id: 1){
             __typename
         }
     }
   

What is this change?

• During the construction of SqlQueryStructure, if it is observed that there are no columns selected, then one of the primary keys is added so that the eventual database query constructed contains a valid column name in the SELECT statement.

How was this tested?

• Integration Tests
• Manual Tests

[Aniruddh25]

Waiting for few answers, perhaps the optimization of not hitting the db can be a followup PR

[aaronpowell]

Waiting for few answers, perhaps the optimization of not hitting the db can be a followup PR

I would have expected that to be the better approach, since you don't actually need any data back from the DB, but I can see there being a problem when you've done a query like:

query {
  books {
    items {
      __typename
    }
  }
}

That should return a items array that has n __typename properties in it, where n is the count of records in the query (up to 100).

[Aniruddh25]

LGTM after 1 optimization to generate expected output

[Aniruddh25]

LGTM. Thanks for addressing all comments.

[ayush3797]

LGTM!