Add field using eval to existing data model field

[tromero3]

Hello! I am just starting to work with data models (and eval expressions) and I am a bit stuck.  I have a data model titled "Intrusion" with has 2 diff indexes A & B. Index A events includes a field called "Signature" in the events, but Index B does not. Index B has a field titled "msg" which is similar to the signature field, so I want to add the msg field to the signature field in the data model. 

Currently the data model has "signature" as an eval expression field with the below expression and field settings.  How can I edit the eval expression so that the "msg" field from Index B shows up as the "signature" field in the data model? Or do I need to add a new field/eval expression completely?  Thank you!

Eval Expression:
`if(isnull(signature) OR signature="","unknown",signature)`

Field Name: signature
Display Name: signature
Type: string
Flags: optional

[anilchaithu]

@tromero3 

A solution would be to rename "msg" field as "Signature". This is called Normalization and is used extensively in to make data CIM compatible..