Trigger scenario 3 in Salesforce:

Using Trigger to populate the account type on the contact record

trigger triggername on contact(before update,before insert){

     if(trigger.isbefore && (trigger.isupdate|| trigger.isinsert){

    Set<Id> accountIds = new Set<Id>();

         for(contact ct: Trigger.new){

       accountIds.add(ct.id);

         }
if(!accids.isempty()){
 map<id,account> mapacc = new map<id,account>([select id,name ,type from account where id IN: accountids]);
for(contact con : trigger.new){
  if(con.accountid != null){
    con.acctype__c = mapacc.get(con.accountid).type;
}
}
}

} 

Trigger to update parent object value with child value and recursive trigger and updating same parent object not child object:

Note:below trigger it doesn't work in this trigger.

trigger AcctoOppTrigger on Account (after update) {

 list<account> lstacc1 = new list<account>();

     if(trigger.isafter && trigger.isupdate){

    Set<Id> accountIds = new Set<Id>();

         for(Account ct: Trigger.new){

       accountIds.add(ct.id);

         }

     list<account> lstacc =  [select id,name,email__c,(select id,email, lastname, Level__c,accountid from contacts where Level__c ='primary') from account where id in : accountIds];

   for(Account ct: lstacc){

       for(contact con : ct.contacts){

           ct.Email__c = con.Email;

             lstacc1.add(ct);

       }

   }

       //  if(recusrssionPreventController.flag == true){

       //    recusrssionPreventController.flag = false;

             if(lstacc1.size()>0)

              update lstacc1;

      // }

     }

}

Note:

With out this below lines it should get below error:

 if(recusrssionPreventController.flag == true){

          recusrssionPreventController.flag = false;

             if(lstacc1.size()>0)

              update lstacc1;

       }

Error: Invalid Data. 
Review all error messages below to correct your data.
Apex trigger AcctoOppTrigger caused an unexpected exception, contact your administrator: AcctoOppTrigger: execution of AfterUpdate caused by: System.DmlException: Update failed. First exception on row 0 with id 0012800000exmUvAAI; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, AcctoOppTrigger: maximum trigger depth exceeded Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv] Account trigger event AfterUpdate for [0012800000exmUv]: []: Trigger.AcctoOppTrigger: line 18, column 1

Note:

If filed value 'level__c'is 'primary' have in 2 contact records associated parent it will one more error like below: 

Error: Invalid Data. 
Review all error messages below to correct your data.
Apex trigger AcctoOppTrigger caused an unexpected exception, contact your administrator: AcctoOppTrigger: execution of AfterUpdate caused by: System.ListException: Duplicate id in list: 0012800000exmUvAAI: Trigger.AcctoOppTrigger: line 18, column 1


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trigger BestPracticesforaccountandcon on account (after insert,after update) 

{

    

  

  //This queries all Contacts related to the incoming Account records in a single SOQL query.

  //This is also an example of how to use child relationships in SOQL

  List<Account> accountsWithContacts = [select id, name,email__c, (select id, salutation, description, 

                                                                firstname, lastname, email from Contacts) 

                                                                from Account where Id IN :Trigger.newMap.keySet()];

  

  List<Contact> contactsToUpdate = new List<Contact>{};

  // For loop to iterate through all the queried Account records 

  for(Account a: accountsWithContacts){

     // Use the child relationships dot syntax to access the related Contacts

     for(Contact c: a.Contacts){

      System.debug('Contact Id[' + c.Id + '], FirstName[' + c.firstname + '], LastName[' + c.lastname +']');

      c.Description=c.salutation + ' ' + c.firstName + ' ' + c.lastname;

         c.Email = a.email__c;

      contactsToUpdate.add(c);

     }      

   }

      

   //Now outside the FOR Loop, perform a single Update DML statement. 

   update contactsToUpdate;

  /*  if(Trigger.isafter)

    {

        list<contact> allcontacts = new list<contact>();

        if(trigger.isinsert)

        {

            for(integer i = 0;i<Trigger.new.size();i++)

            {

                contact con = new contact();

                con .LastName= Trigger.new[i].name;

              // con .LastName =con .LastName + 'devi';

                con .Email= Trigger.new[i].Email__c;

                con .Department= Trigger.new[i].AccountNumber;

                con .Accountid = Trigger.new[i].id;

                allcontacts.add(con);

             } 

               insert allcontacts;

          }

        

        list<id> allAccountIDs = new list<id>();

        map<id,account> allAccounts = new Map<id,account>();

        if(Trigger.isupdate)

        {

                for(integer i = 0;i<Trigger.new.size();i++)

           {

               allAccountIDs.add(Trigger.new[i].id);

               allAccounts.put(Trigger.new[i].id,Trigger.new[i]);

            }

            list<contact> listcon = [select id,LastName,Email,Department,AccountID  from contact where Accountid =:allAccountIDs];    

            for(integer i =0;i<listcon.size();i++)

            {

                if(allAccounts.containskey(listcon[i].AccountID ))

                {

                     // listcon[i].LastName =listcon[i].LastName+'devi';

                    listcon[i].LastName = allAccounts.get(listcon[i].AccountID ).name;

                    listcon[i].Email = allAccounts.get(listcon[i].AccountID ).Email__c;

                    listcon[i].phone = allAccounts.get(listcon[i].AccountID ).phone;

                    listcon[i].Department = allAccounts.get(listcon[i].AccountID ).AccountNumber;

                    listcon[i].Accountid = allAccounts.get(listcon[i].AccountID).id;

                    allcontacts.add(listcon[i]);

                }

            }

                 update allcontacts;

        }    

    } */

}