yii\data\BaseDataProvider::prepare PHP Method

    public function prepare($forcePrepare = false)
    {
        if ($forcePrepare || $this->_models === null) {
            $this->_models = $this->prepareModels();
        }
        if ($forcePrepare || $this->_keys === null) {
            $this->_keys = $this->prepareKeys($this->_models);
        }
    }