fix(json-columns): skip JSON select columns with no model scan target

Requesting a JSON sub-field (e.g. jsonvalue->'product'->>'cost') that has
no matching bun scanonly field on the model made the whole read fail with
"bun: ModelX does not have column Y", since bun scans SELECT results
straight into the typed model struct.

Add reflection.HasColumn to check whether the model can actually receive
a given column (including scanonly fields, walking embedded structs), and
gate the JSON select-column expression on it in ApplySelectColumns
(shared by websocketspec/mqttspec) and the resolvespec/restheadspec
handlers. When there's no scan target, drop just that column with a
warning instead of erroring the whole request.

pkg/common/json_condition.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"strings"
 
+	"github.com/bitechdev/ResolveSpec/pkg/logger"
 	"github.com/bitechdev/ResolveSpec/pkg/reflection"
 )
 
@@ -72,6 +73,15 @@ func ResolveJSONColumnExpr(model interface{}, tableAlias, token string) (expr st
 func ApplySelectColumns(query SelectQuery, model interface{}, tableAlias string, columns []string) SelectQuery {
 	for _, col := range columns {
 		if expr, args, alias, ok := ResolveJSONColumnExpr(model, tableAlias, col); ok {
+			if !reflection.HasColumn(model, alias) {
+				// No matching scan target on the model (e.g. no
+				// `bun:"<alias>,scanonly"` field declared for this JSON
+				// path) - bun would fail to scan the row with "does not
+				// have column X". Drop the expression rather than erroring;
+				// the rest of the requested columns still get selected.
+				logger.Warn("Skipping JSON select column %q: model has no scan target for alias %q", col, alias)
+				continue
+			}
 			query = query.ColumnExpr(expr+" AS "+QuoteIdent(alias), args...)
 			continue
 		}

pkg/common/json_condition_test.go

@@ -2,6 +2,7 @@ package common
 
 import (
 	"reflect"
+	"strings"
 	"testing"
 
 	"github.com/bitechdev/ResolveSpec/pkg/spectypes"
@@ -162,3 +163,43 @@ func TestBuildJSONFilterCondition_QualifiedAndInjectionSafe(t *testing.T) {
 		t.Errorf("args = %#v", args)
 	}
 }
+
+// selectCapQuery is a minimal SelectQuery that records Column/ColumnExpr calls
+// so ApplySelectColumns' behaviour can be asserted without a real DB.
+type selectCapQuery struct {
+	SelectQuery
+	columns     []string
+	columnExprs []string
+}
+
+func (m *selectCapQuery) Column(cols ...string) SelectQuery {
+	m.columns = append(m.columns, cols...)
+	return m
+}
+func (m *selectCapQuery) ColumnExpr(q string, args ...interface{}) SelectQuery {
+	m.columnExprs = append(m.columnExprs, q)
+	return m
+}
+
+// jsonSelectModel has a real JSON column (Data) but only ONE pre-declared
+// scanonly field for a computed JSON path ("data_city"); "data_age" has no
+// matching scan target.
+type jsonSelectModel struct {
+	ID       int64              `json:"id" bun:"id,pk"`
+	Data     spectypes.SqlJSONB `json:"data" bun:"data"`
+	DataCity string             `json:"-" bun:"data_city,scanonly"`
+}
+
+func TestApplySelectColumns_SkipsJSONColumnWithoutScanTarget(t *testing.T) {
+	m := jsonSelectModel{}
+	q := &selectCapQuery{}
+
+	ApplySelectColumns(q, m, "", []string{"id", "data.city", "data.age"})
+
+	if !reflect.DeepEqual(q.columns, []string{"id"}) {
+		t.Errorf("columns = %#v, want [id]", q.columns)
+	}
+	if len(q.columnExprs) != 1 || !strings.Contains(q.columnExprs[0], `AS "data_city"`) {
+		t.Errorf("columnExprs = %#v, want exactly one expr aliased data_city", q.columnExprs)
+	}
+}