Relationships in Apex

Relationships in Apex

Relationship Queries 

Child-to-Parent relationship

Using the relationships, we can retrive the related Objects data using the SOQL query.

Contacts --------------<-----------Account

Relationship name = Account

Child to parent Relationship

In the child -to-parent Relationship, we can query on Contact and retrieves the values from Account that is associated.

Ex:

Contact C = [Select First Name, LastName, Account.Name, Account.Industry from Contact where id = 'XXXXX'];

System.debug (c. Account .Name); --->Prints the Account Name of Contact

System.debug (C. Account. Industry); ---> Prints the industry field value from account

Parent to child relationship

In the parent-to-child relationship. We can query on the parent object and we will get the details about child records.

EX:

Account a = [Select Name, (Select Contact .FirstName, contact .LastName from Account. Contacts) From Account where id ='xxxxxxx']

System debug (a); àthis will give all the Contact for that account

For (Contact C: a.Contacts)

{

system.debug (C. First Name);

}

This statement displays FirstName of all contacts that are associated with the account.When we want to write query on parent to child relationship then we need to write subquery ( query within another query)

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Custom Relationships:-

This selection explains how relationship names for Custom objects and Custom fields that are created and used

*When we use a relationship name ina query. We must use the relationship
names without __c.instead, append an---r(underscore underscore r).
Daughter__c------------------->--------Mother-of-child__c
Relationship Name =Mother -of-child---r, child -to-parent Relationship.

parent-to-child Relationship, Relationship =Daughter--r

When we use a child-to-parent relationship.

EX:- Daughter_ _c D= [Select id, First name _ _C, Mother-of child_ _r.firstname from daughter where LastName_ _C LIKE 'C%']

The above query returns the ID and first name of daughter objects, and the first name of the daughter's mother if the mother’s last Name begins with 'C'

EX:- SELECT LastName __c, (Select LastName__c From Daughters --r) FROM mother- of-child __c.

The above query returns the Last name of all mothers, and for each mother returned, the last name of the mother's daughters.

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