Making Apache Cayenne respect default values in Postgres

SetNewObjectDefaults.java

/**
 * Modified from http://objectstyle.org/cayenne/lists/cayenne-user/2007/12/0084.html
 * for use with Postgres. However, it performs many SELECTs which may or may not be
 * efficient depending on your application.
 */
private void setNewObjectDefaults() {
    BaseDataObject mainObj = (BaseDataObject) this;
    ObjEntity ent = Cayenne.getObjEntity(mainObj);
    Collection attribs = ent.getAttributes();

    // Try and get the default values from the database
    String defaultSql = getDefaultValueLookupQuery(ent.getDbEntityName());
    SQLTemplate rawSelect = new SQLTemplate(mainObj.getClass(), defaultSql);
    rawSelect.setFetchingDataRows(true);

    // Now get a list of column names and results.
    List defList = mainObj.getObjectContext().performQuery(rawSelect);

    // Loop over the attributes that the mainObj has.
    for (int i = 0; i <= attribs.size() - 1; i++) {
        ObjAttribute attr = (ObjAttribute) attribs.toArray()[i];

        // We need to find something out about this attribute
        DbAttribute att = attr.getDbAttribute();
        String propName = attr.getName();
        Object propertyValue = mainObj.readProperty(propName);
        Class propertyType = attr.getJavaClass();
        Object def = null;

        // Do not process if there is already a value
        // in the object's attributes
        if (propertyValue != null) {
            continue;
        }

        // Now that we're looping over the attributes,
        // we should also loop over the default values returned above
        // and check if they are both in each others' list
        for (int j = 0; j <= defList.size() - 1; j++) {
            Map row = (Map) defList.get(j);
            String rowFieldName = (String) row.get("column_name");

            // If they are in each others' list then set the column default
            // as the default object
            if (rowFieldName.equals(att.getName())) {
                def = row.get("column_default");
                break;
            }
        }

        Object result = getDefaultValue(def, mainObj.getObjectContext());

        // If the value is null then write null
        if (result == null) {
            mainObj.writeProperty(propName, null);
            continue;
        }

        // Here we check what class the result is.
        // The result may be a Date or a String or anything defined
        // by SQLDataTypes, so we have to check what type it is below.
        Class<?> keyClass = result.getClass();

        // TODO: This may need modifying depending on what you get returned
        if (keyClass.equals(Date.class)) {
            Date d = (Date) result;
            String ds = d.toString();
            DateTimeFormatter dtf = DateTimeFormatter.ofPattern("EEE MMM dd HH:mm:ss zzz yyyy");
            LocalDateTime l = LocalDateTime.parse(ds, dtf);
            mainObj.writeProperty(propName, l);
        } else if (keyClass.equals(Long.class)) {
            mainObj.writeProperty(propName, result);
        } else if (keyClass.equals(Integer.class)) {
            mainObj.writeProperty(propName, result);
        } else {
            // Just try writing - warning this may error
            // because I haven't written in all the classes
            // that the default value may return
            mainObj.writeProperty(propName, result);
        }

    }
}

/**
 * Because the defaultValue may be an expression, here what we're doing
 * is SELECTing that expression to return it.
 * @param defaultValue The default value returned from a query, should be a String
 * @param context An ObjectContext to perform finding
 * @return The actual default value
 */
public Object getDefaultValue(Object defaultValue, ObjectContext context) {
    SQLTemplate sel = new SQLTemplate("SELECT " + defaultValue, true);
    List query = context.performQuery(sel);

    // This returns a one column table with for example "now"
    // or "int8" as the heading and the value.
    Map table = (Map) query.get(0);

    // Grab that one column's entry
    java.util.HashMap.Entry firstCol = (java.util.HashMap.Entry) table.entrySet().toArray()[0];

    // .. and then return the value
    return firstCol.getValue();
}

protected String getDefaultValueLookupQuery(String entName) {
    String def = "SELECT column_name, column_default " +
        "FROM information_schema.columns " +
        "WHERE (table_schema, table_name) = ('public', '" +  entName + "') " +
        "ORDER BY ordinal_position;";
    return def;
}