I'm not sure how to do this as I don't' think there are any ModelBuilder tools for this task. I'm not real savvy with Python but basically I want to relate field1 in table A to field2 in table B with a one to many relationship. So I want all records in table B that match Table A.
The Add Join (Data Management) tool
Joins a layer to another layer or table (where layer is a feature layer, table view, or raster layer with a raster attribute table) based on a common field.
This can be used in ModelBuilder and supports both one-to-one and many-to-one - if you need one-to-many you should be able to reverse the order of your inputs.
"Map Rantala" came up with this when Trying to perform “relate” in Model Builder. It uses the Make Table tool to perform a relate like operation.