NOTE
The information contained in this document represents information about preview features of the product. Features might change when the product is released for general availability.
Direct Access Classes API
The Direct Access Classes model the tables in the database.
For each table in the database, gaiac generates code for a Direct Access Class (DAC) that includes the following items.
Direct Access Classes implements reader and writer classes that provide zero-copy reads and direct references to related records. Since the Gaia database resides in memory, accessing a record is the same as accessing a local variable. This eliminates the need to cache database values in the client code.
Accessors
The DAC implements an accessor for each field in the table to retrieve the value of the field. For example:
const char* doctor_t::field_name1() constuint64_t doctor_t::field_name2() constfloat doctor_t::field_name3() constbool doctor_t::field_name4() const
For information on the full set of supported types, see create table.
Examples in these articles are taken from the Direct Access sample app that is installed with the SDK at /opt/gaia/examples/direct_access.
Constructors
doctor_t:doctor_t()
Public default constructor. Creates an empty DAC instance.
It is valid to cache DAC across transaction boundaries. The system guarantees that the correct data is maintained.
Methods
insert_row
gaia::common::gaia_id_t doctor_t::insert_row(<field 1>, <field2>,...)
Adds a row to the table described by the class.
gaia_typename
const char* doctor_t::gaia_typename()
Returns the class name doctor_t.
list
gaia::direct_access::edc_container_t<c_gaia_type_address, doctor_t doctor_t::list()
Exposes an iterator over all the records in a table.
Structure for filtering
Provides predicates that you use to filter rows using the container class predicates.
Example
It this example doctor is the class name that replaces [class name] in the template above.
struct expr_ {
static gaia::direct_access::expression_t\<doctor_t, gaia::common::gaia_id_t gaia_id;
static gaia::direct_access::expression_t<doctor_t, const char* street;
static gaia::direct_access::expression_t<doctor_t, const char* city;
static gaia::direct_access::expression_t<doctor_t, patient_t patient;
};
In addition, the DAC exposes additional methods that enable you to work with table data.
gaia_type
gaia::common::gaia_type_t gaia_type() override;
Returns the type id for the DAC.
get
static T_gaia get(gaia::common::gaia_id_t id);
Retrieves a specific object of the type T_gaia based on its ID. An exception is thrown if the object represented by the ID is not of type T_gaia.
@param ID the gaia_id_t of a specific database object, of type container_type_id.
delete_row
void delete_row();
Delete the database object. This doesn't destroy the extended data class object.
static void delete_row(gaia::common::gaia_id_t id);
Delete the database object specified by the ID.
has_value
has_value();
Returns a boolean indicating whether there is a value in the field.
writer
doctor_t writer();
Returns a reference that is pre-populated with values from the row.
Writer Class API
doctor_writer (as a typedef in the generated code)
The writer class provides methods that allow you to add, update, and delete rows from DAC. This includes setters for each field in the class.
delete_row
doctor_t::delete_row()
Deletes the current row pointed to by the Direct Access doctor_t object.
The following code snippet deletes all the values in the table doctor.
for (auto doctor_it = doctor_t::list().begin();
doctor_it != doctor_t::list().end();)
{
auto next_doctor_it = doctor_it++;
next_doctor_it->delete_row();
}
insert_row
doctor_writer::insert_row
Adds a new row in a Direct Access *class name*_t object.
update_row
doctor_writer::update_row
Updates current row pointed to by a DA_t object.
Example
The following code snippet inserts a new patient and then update his information after the row was inserted.
gaia_id_t id = patient_t::insert_row("John", 175, false, nullptr, {});
patient_t john = patient_t::get(id);
gaia_log::app().info("Patient name is: {}", john.name());
// Obtain the writer object for an existing record.
patient_writer john_w = john.writer();
john_w.name = "Jane";
john_w.height = 178;
john_w.analysis_results = {1.0, 1.2, 0.3};
john_w.is_active = true;
john_w.update_row();
gaia_log::app().info("Patient name now is: {}", john.name());