Nested Document Fields

When accessing nested document fields, it is not possible to obtain them directly through lambda expressions. Therefore, MongoPlus provides alternative ways to build nested document fields, avoiding the need to write them as plain strings.
Nested document fields, also known as embedded documents, are for cases like when the User class has a role property of type Role, and you want the value "role.roleName". This can be achieved using this approach.

FieldChain

• This class allows you to build nested document fields, making it more convenient than FunctionUtil.
• You can also retrieve field names prefixed with $.

Example

Get nested fields

public class User {
    private Role role;
}
public class Role {
    private String roleName;
}

public static void main(String[] args) {
    FieldChain fieldChain = FieldChain.of(User::getRole).then(Role::getUserName);
    // Build the field name using build()
    String fieldName = fieldChain.build();
    // Output: role.roleName
    System.out.println(fieldName);

    // -------------------------------------------------------------------

    // Build field name with $
    String fieldNameOption = fieldChain.build(true);
    // Output: $role.roleName
    System.out.println(fieldNameOption);
}

---

FunctionUtil

• This class can also be used to build nested document fields.
• It is not limited to nested fields — it can also handle normal top-level fields.
• You can also retrieve field names prefixed with $.

Example

Get top-level fields

public class User {
    private String userName;
}

public static void main(String[] args) {
    // Get the field name. If @CollectionField annotation specifies a field name, that value will be returned.
    String fieldName = FunctionUtil.getFieldName(User::getUserName)
    // Output: userName
    System.out.println(fieldName);

    // -------------------------------------------------------------------

    // Get the field name prefixed with $
    String fieldName = FunctionUtil.getFieldNameOption(User::getUserName)
    // Output: $userName
    System.out.println(fieldName);
}

Get nested fields

public class User {
    private Role role;
}
public class Role {
    private String roleName;
}

public static void main(String[] args) {
    // Build a nested document object
    FunctionBuilder builder = FunctionUtil.builderFunction();
    // Add fields to the FunctionBuilder object
    builder.add(User::getRole).add(Role::getRoleName);
    // Build the field name
    String fieldName = builder.build();
    // Output: role.roleName
    System.out.println(fieldName);

    // -------------------------------------------------------------------

    // Build field name with $
    String fieldNameOption = builder.build(true);
    // Output: $role.roleName
    System.out.println(fieldNameOption);
}