Zend Anwar

Q. How will you log current collection’s SQL query?

A. $collection->printLogQuery(true); OR $collection->getSelect()->__toString();

Q. How to get first item or last item from the collection?

A. $collection->getFirstItem() and $collection->getLastItem();

Q. Where is the relation between configurable product and it’s simple product stored in database?

A.
In the 2 tables:
catalog_product_relation
catalog_product_superlink_table

Q. What are the commonly used block types? What is the special in core/text_list block type.

A.
Commonly used block types: core/template, page/html, page/html_head, page/html_header, page/template_links, core/text_list, page/html_wrapper, page/html_breadcrumbs, page/html_footer, core/messages, page/switch.
Some blocks like content, left, right etc. are of type core/text_list. When these blocks are rendered, all their child blocks are rendered automatically without the need to call getChildHtml() method.

Q. Explain different types of sessions in Magento (e.g. customer/session, checkout/session, core/session) and the reason why you store data in different session types?

A.
Customer sessions stores data related to customer, checkout session stores data related to quote and order. They are actuall under one session in an array. So firstname in customer/session will be $_SESSION[‘customer’][‘firstname’] and cart items count in checkout/session will be $_SESSION[‘checkout’][‘items_count’]. The reason Magento uses session types separately is because once the order gets placed, the checkout session data information should get flushed which can be easily done by just unsetting $_SESSION[‘checkout’] session variable. So that the session is not cleared, just session data containing checkout information is cleared and rest all the session types are still intact.

Q. Where will you write your module’s business logic in Magento?

A. inside Model.

Q. How many database tables will Magento create when you make a new EAV module?

A.
Magento creates 6 tables when you create new EAV module. Tables: module, module_datetime, module_decimal, module_int, module_text and module_varchar. one is the main entity table, and rest 5 tables which holds attribute’s data in different data types. So that integer values will go to module_int table, price values to module_decimal, etc.

Q. Can you have more than one Grid in a module?

A. Yes.

Q. Difference between EAV and flat model.

A.
EAV is entity attribute value database model, where data is fully in normalized form. Each column data value is stored in their respective data type table. Example, for a product, product ID is stored in catalog_product_entity_int table, product name in catalog_product_entity_varchar, product price in catalog_product_entity_decimal, product created date in catalog_product_entity_datetime and product description in catalog_product_entity_text table. EAV is complex as it joins 5-6 tables even if you want to get just one product’s details. Columns are called attributes in EAV.

Flat model uses just one table, so it’s not normalized and uses more database space. It clears the EAV overhead, but not good for dynamic requirements where you may have to add more columns in database table in future. It’s good when comes to performance, as it will only require one query to load whole product instead of joining 5-6 tables to get just one product’s details. Columns are called fields in flat model.

Q. How will you call a CMS page in your module’s PHTML file?

A. $this->getLayout()->createBlock(‘cms/block’)->setBlockId(‘blockidentifier’)->toHtml();
Q. How Magento ORM works?

A.
ORM stands for Object Relational Mapping. It’s a programming technique used to convert different types of data to Objects and vice versa.

In Magento, ORM is shown as Model (based on Zend Framework’s Zend_Db_Adapter), which further breaks down to two types of Models.

– First is the “simple” i.e. Regular Models which is nothing but flat table or our regular table structure.
– Second Model is EAV (Entity Attribute Value), which is quite complicated and expensive to query.

All Magento Models interacting with database are inherited from Mage_Core_Model_Abstract class, which is further inherited from Varien_Object.

Difference between two Models is, Simple Model is inherited from Mage_Core_Model_Resource_Db_Abstract class,
while EAV is inherited from Mage_Eav_Model_Entity_Abstract.

For those who don’t know what EAV is, please read my 3rd answer below.
So, to end up this question,
when you want to get some data in Magento, you call it like this:

Mage::getModel(‘module/model’)->load(1);

where 1 is the primary key id for some Regular/Simple table, while in EAV so many tables are joined to fetch just single row of data.