Constant SEARCH_ISSUES 

pub const SEARCH_ISSUES: &str = "query($search:String!,$type:SearchType!,$first:Int!,$after:String,$nestedFirst:Int!,$boardItems:Int!,$duplicates:Boolean!){\n      search(query:$search,type:$type,first:$first,after:$after){\n        pageInfo{hasNextPage endCursor}\n        nodes{__typename ...BoardIssue}\n      }\n    } fragment BoardIssue on Issue{__typename id title body url createdAt updatedAt state stateReason(enableDuplicate:$duplicates) repository{nameWithOwner} parent{id} subIssuesSummary{total}\n      labels(first:$nestedFirst){nodes{id name color}pageInfo{hasNextPage}}\n      projectItems(first:$boardItems){nodes{id project{number}\n        fieldValues(first:$nestedFirst){nodes{\n          ... on ProjectV2ItemFieldSingleSelectValue{name field{\n            ... on ProjectV2SingleSelectField{id name options{id name}}\n          }}\n          ... on ProjectV2ItemFieldTextValue{text field{... on ProjectV2Field{id name}}}\n        }pageInfo{hasNextPage}}}pageInfo{hasNextPage}}}";

Every issue of one board, found by a search scoped to that board.

This is how the projects a board holds are listed, and it selects no items connection on ProjectV2: the board is a qualifier of the search rather than a container walked page by page, so nothing nested inside a board item is paid for. Which of the issues it returns is a project is then read off parent — GitHub accepts -has:parent as a search qualifier and silently ignores it, so the discriminator has to be applied to the field, which is a scalar on the issue and costs nothing.