feat(restheadspec): match NULL when x-fieldfilter compares a numeric column to 0

Adds a SetNullAsZero handler option: on by default in restheadspec (for
x-fieldfilter only), off by default in resolvespec. When enabled, an eq
filter of 0 on a numeric column renders (col = 0 OR col IS NULL).

pkg/resolvespec/handler.go

@@ -33,6 +33,7 @@ type Handler struct {
 	openAPIGenerator func() (string, error)
 	defaultSort      map[string][]common.SortOption
 	disallowNulls    bool
+	nullAsZero       bool
 }
 
 // NewHandler creates a new API handler with database and registry abstractions
@@ -61,6 +62,12 @@ func (h *Handler) SetDisallowNulls(disallow bool) {
 	h.disallowNulls = disallow
 }
 
+// SetNullAsZero controls whether an equality filter against 0 on a numeric
+// column also matches NULLs: (col = 0 OR col IS NULL). Disabled by default.
+func (h *Handler) SetNullAsZero(enabled bool) {
+	h.nullAsZero = enabled
+}
+
 // SetFallbackHandler sets a fallback handler to be called when no model is found
 // If not set, the handler will simply return (pass through to next route)
 func (h *Handler) SetFallbackHandler(fallback FallbackHandler) {
@@ -2026,7 +2033,11 @@ func (h *Handler) buildFilterConditionAlias(filter common.FilterOption, model in
 
 	switch filter.Operator {
 	case "eq", "=":
-		condition = fmt.Sprintf("%s = ?", filter.Column)
+		if h.nullAsZero && common.IsNumericZeroFilter(model, rawColumn, filter.Value) {
+			condition = fmt.Sprintf("(%s = ? OR %s IS NULL)", filter.Column, filter.Column)
+		} else {
+			condition = fmt.Sprintf("%s = ?", filter.Column)
+		}
 		args = []interface{}{filter.Value}
 	case "neq", "!=", "<>":
 		condition = fmt.Sprintf("%s != ?", filter.Column)
@@ -2111,7 +2122,11 @@ func (h *Handler) applyFilter(query common.SelectQuery, filter common.FilterOpti
 
 	switch filter.Operator {
 	case "eq", "=":
-		condition = fmt.Sprintf("%s = ?", filter.Column)
+		if h.nullAsZero && common.IsNumericZeroFilter(model, rawColumn, filter.Value) {
+			condition = fmt.Sprintf("(%s = ? OR %s IS NULL)", filter.Column, filter.Column)
+		} else {
+			condition = fmt.Sprintf("%s = ?", filter.Column)
+		}
 		args = []interface{}{filter.Value}
 	case "neq", "!=", "<>":
 		condition = fmt.Sprintf("%s != ?", filter.Column)