How to create a multi-select lookup relationship between two custom objects?

Hello Hazel,

Below is the code that works for the multiselect lookup between the 2 objects. I have used it for Standard objects Account and Contact, you can modify it as per your use for the 2 custom objects.

Below is my code for the vf page:-

<apex:page standardController="Account" extensions="customMutliSelectLookUpCntrl">
<style type="text/css">
.customPopup {
background-color: white;
border-style: solid;
border-width: 2px;
left: 20%;
padding: 10px;
position: absolute;
z-index: 9999;
width: 500px;
top: 20%;
}

.disabledTextBox {
background-color: white;
border: 1px solid;
color: black;
cursor: default;
width: 90px;
display: table;
padding: 2px 1px;
text-align:right;
}

.closeButton {
float: right;
}
</style>
<apex:form >
<apex:pageBlock id="counter">
<apex:inputtextarea value="{!lookUp}" label="Contact"/>
<apex:commandButton value="Add" reRender="out" action="{!add}"/>
<apex:outputPanel id="out">
<apex:outputPanel styleClass="customPopup" rendered="{!bool}">
<apex:commandButton value="X" title="Close the popup" action="{!closePopup}" styleClass="closeButton" rerender="out"/>
<apex:pageBlockTable value="{!show}" var="e" title="show">
<apex:column >
<apex:inputCheckbox value="{!e.check}"/>
<apex:actionSupport event="onclick" action="{!inIt}"/>
</apex:column>
<apex:column >
<apex:commandLink value="{!e.con.Name}"/>
</apex:column>
</apex:pageBlockTable>
</apex:outputPanel>
</apex:outputPanel>
<apex:commandButton value="Save" action="{!mySave}"/>
</apex:pageBlock>
</apex:form>
</apex:page>
So once you have created the above page just use the below apex class code :-

public with sharing class customMutliSelectLookUpCntrl{

public string lookUp{get;set;}
public list<conContact> contactList{get;set;}
public boolean allbool{get;set;}
public string inputValue{get;set;}
public boolean bool{get;set;}
public set<id> contactids{get;set;}
ApexPages.StandardController controller;

public customMutliSelectLookUpCntrl(ApexPages.StandardController con){
controller = con;
contactList = new list<conContact>();
bool = false;
contactids = new Set<Id>();
}

public class conContact{

public contact con{get;set;}
public boolean check{get;set;}

public conContact(contact c, boolean boo){
con = c;
check = boo;
}
}

public void inIt(){

list<Contact> selectedContact = new list<Contact>();
lookUp = '';
for(conContact conObj : contactList){
if(conObj.check != false){
system.debug('conObj.con'+conObj.con);
selectedContact.add(conObj.con);
lookUp += conObj.con.name + ' ';
system.debug('lookUp::'+lookUp);
contactids.add(conObj.con.id);
}
}
bool = true;
}
public list<conContact> getShow(){

for(Contact coObj:[select id,name from Contact]){
contactList.add(new conContact(coObj,allbool));
}
return contactList;

}

public PageReference mySave(){

list<Contact> updateSelectedContact = new list<Contact>();
id accId = ApexPages.CurrentPage().getparameters().get('id');
for(Contact co:[select id,name,accountid from Contact where id =: contactids]){
co.accountid = accId;
updateSelectedContact.add(co);
}
update updateSelectedContact;
return null;
}

public void closePopup()
{
bool = false;
}

public void add(){
bool = true;

}
}

Once you have modified the code according to your requirement and the vf page as inline vf page to the layout of the record.

Thanks.