feat: add CLI command for importing and exporting Pocket ID data (#998)

Co-authored-by: Alessandro (Ale) Segala <43508+ItalyPaleAle@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

backend/internal/utils/db_util.go

@@ -0,0 +1,116 @@
+package utils
+
+import (
+	"fmt"
+	"strings"
+
+	"gorm.io/gorm"
+)
+
+// DBTableExists checks if a table exists in the database
+func DBTableExists(db *gorm.DB, tableName string) (exists bool, err error) {
+	switch db.Name() {
+	case "postgres":
+		query := `SELECT EXISTS (
+			SELECT FROM information_schema.tables 
+			WHERE table_schema = 'public' 
+			AND table_name = ?
+		)`
+		err = db.Raw(query, tableName).Scan(&exists).Error
+		if err != nil {
+			return false, err
+		}
+	case "sqlite":
+		query := `SELECT COUNT(*) > 0 FROM sqlite_master WHERE type='table' AND name=?`
+		err = db.Raw(query, tableName).Scan(&exists).Error
+		if err != nil {
+			return false, err
+		}
+	default:
+		return false, fmt.Errorf("unsupported database dialect: %s", db.Name())
+	}
+
+	return exists, nil
+}
+
+type DBSchemaColumn struct {
+	Name     string
+	Nullable bool
+}
+type DBSchemaTableTypes = map[string]DBSchemaColumn
+type DBSchemaTypes = map[string]DBSchemaTableTypes
+
+// LoadDBSchemaTypes retrieves the column types for all tables in the DB
+// Result is a map of "table --> column --> {name: column type name, nullable: boolean}"
+func LoadDBSchemaTypes(db *gorm.DB) (result DBSchemaTypes, err error) {
+	result = make(DBSchemaTypes)
+
+	switch db.Name() {
+	case "postgres":
+		var rows []struct {
+			TableName  string
+			ColumnName string
+			DataType   string
+			Nullable   bool
+		}
+		err := db.
+			Raw(`
+				SELECT table_name, column_name, data_type, is_nullable = 'YES' AS nullable
+				FROM information_schema.columns
+				WHERE table_schema = 'public';
+			`).
+			Scan(&rows).
+			Error
+		if err != nil {
+			return nil, err
+		}
+		for _, r := range rows {
+			t := strings.ToLower(r.DataType)
+			if result[r.TableName] == nil {
+				result[r.TableName] = make(map[string]DBSchemaColumn)
+			}
+			result[r.TableName][r.ColumnName] = DBSchemaColumn{
+				Name:     strings.ToLower(t),
+				Nullable: r.Nullable,
+			}
+		}
+
+	case "sqlite":
+		var tables []string
+		err = db.
+			Raw(`SELECT name FROM sqlite_master WHERE type='table' AND name NOT LIKE 'sqlite_%';`).
+			Scan(&tables).
+			Error
+		if err != nil {
+			return nil, err
+		}
+		for _, table := range tables {
+			var cols []struct {
+				Name    string
+				Type    string
+				Notnull bool
+			}
+			err := db.
+				Raw(`PRAGMA table_info("` + table + `");`).
+				Scan(&cols).
+				Error
+			if err != nil {
+				return nil, err
+			}
+			for _, c := range cols {
+				if result[table] == nil {
+					result[table] = make(map[string]DBSchemaColumn)
+				}
+				result[table][c.Name] = DBSchemaColumn{
+					Name:     strings.ToLower(c.Type),
+					Nullable: !c.Notnull,
+				}
+			}
+		}
+
+	default:
+		return nil, fmt.Errorf("unsupported database dialect: %s", db.Name())
+	}
+
+	return result, nil
+}