Visualforce Apex Trigger updating wrong records - triggers

Question

I cant figure out why this trigger is sometimes updating records that dont match the criteria. The idea is that when an account goes from 'on-hold' to an active service, any cancelled assignments are returned to pending. I cant figure out whats triggering it, but it seems everyone in a while, assignments are un-cancelled for accounts that have no change in service. Heres the code:
trigger cancelAssignments on Account (before update) {
List<Assignment__c> masterListA = [select Id, Status__c, Practice__c from Assignment__c where Practice__c IN :Trigger.newMap.keySet() and type_of_work__c != 'a0Qa000000G1WmVEAV' AND (status__c = 'Feedback Needed' OR status__c = 'Pending Review' OR status__c = 'Accepted')];
List<Assignment__c> masterListB = [select Id, Status__c, Practice__c from Assignment__c where Practice__c IN :Trigger.newMap.keySet() and type_of_work__c != 'a0Qa000000G1WmVEAV' AND (status__c = 'Canceled')];
for (Account oAccount : trigger.new) {
if (oAccount.current_services__c == null || oAccount.current_services__c == 'Hold'){
for (Account oAcct : trigger.old){
if (oAcct.current_services__c != null && oAcct.current_services__c != 'Hold'){
List<Assignment__c> assignmentsToUpdate = new List<Assignment__c>();
for (Assignment__c rd : masterListA){
if (rd.practice__c == oAccount.id){
rd.Status__c = 'Canceled';
assignmentsToUpdate.add(rd);
}
}update assignmentsToUpdate;
}
}
}
else if (oAccount.current_services__c != 'Hold' && oAccount.current_services__c != null ){
for (Account oAcctB : trigger.old){
if (oAcctB.current_services__c == 'Hold'){
List<Assignment__c> assignmentsToUpdateB = new List<Assignment__c>();
for (Assignment__c rdB : masterListB){
if (rdB.practice__c == oAccount.id){
rdB.Status__c = 'Pending Review';
assignmentsToUpdateB.add(rdB);
}
}update assignmentsToUpdateB;
}
}
}
}
}

Answer 1

The problem may be as follows:
masterListB grabs assignments for more than one account
The code in the for (Account oAcctB : trigger.old) loop never checks to see whether old account that's in "Hold" status is the same account
One solution may be to make the edit below:
/* Old condition replaced:
if (oAcctB.current_services__c == 'Hold') { */
if (oAcctB.current_services__c == 'Hold' and oAcctB.Id == oAccount.Id) {
To prove whether this is the right solution, I suggest creating a unit test that works like this:
Create two Accounts, "Alpha Corp" and "Beta Corp"
Set Alpha Corp's Current Services to "Hold"
Set Beta Corp's Current Services to "Active"
Add a related Assignment to Alpha Corp with Status "Canceled"
Add a related Assignment to Beta Corp with Status "Canceled"
Update both accounts in a single DML operation, doing something trivial like changing the Billing Country from "US" to "United States"
Assert that the related Assignment for Alpha Corp remains "Canceled"
I suspect that the code you shared will not pass the unit test, in which case you'll be able to zero in on the problem and fix it.

Answer 2

While Marty's code answered the question I asked, I also ran into some 'Too Many Code Statements' errors. Here is the final code that seems to resolve that:
trigger cancelAssignments on Account (before update) {
List<account> quitingAccounts = new List<account>();
List<account> returningAccounts = new List<account>();
List<Assignment__c> assignmentsToCancel;
List<Assignment__c> assignmentsToReturn;
for (Account oAccount : trigger.new)
{
if (oAccount.current_services__c == null || oAccount.current_services__c == 'Hold')
{
for (Account oAcct : trigger.old)
{
if (oAcct.current_services__c != null && oAcct.current_services__c != 'Hold' && oAcct.Id == oAccount.Id)
{
quitingAccounts.add(oAcct);
}
}
}
else if (oAccount.current_services__c != 'Hold' && oAccount.current_services__c != null )
{
for (Account oAcctB : trigger.old)
{
if (oAcctB.current_services__c == 'Hold' && oAcctB.Id == oAccount.Id)
{
returningAccounts.add(oAcctB);
}
}
}
}
assignmentsToCancel = [select Id, Status__c, Practice__c from Assignment__c where Practice__c IN :quitingAccounts and type_of_work__c != 'a0Qa000000G1WmVEAV' AND (status__c = 'Feedback Needed' OR status__c = 'Pending Review' OR status__c = 'Accepted')];
assignmentsToReturn = [select Id, Status__c, Practice__c from Assignment__c where Practice__c IN :returningAccounts and type_of_work__c != 'a0Qa000000G1WmVEAV' AND status__c = 'Canceled'];
for (Assignment__c rd : assignmentsToCancel)
{
rd.Status__c = 'Canceled';
}
for (Assignment__c rd : assignmentsToReturn)
{
rd.Status__c = 'Pending Review';
}
update assignmentsToCancel;
update assignmentsToReturn;
}