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    <p>I realize that this question was about <strong>EF4</strong> but I am sure that many people who are just now "making the switch" will end up here via Google and will read this and the approved answer and make decisions based on it even though they are using EF5 (or EF4.4 if you are stuck on .Net 4.0)</p> <p><strong>EF5</strong> allows <strong>multiple diagrams</strong> per edmx. This is a big deal, at least to my team, because it allows us to visually separate entities without requiring separate edmx files. Dr. Zim's points are all still valid except (obviously) the "cluttered designer surface".</p> <p><strong>There are draw backs to having multiple edmx files,</strong> the biggest one is that even if you create separate namespaces for each, you cannot duplicate entity names. Yes, if you truly are designing your system "code first" then this should not be a problem. However, many (most) of us are adding EF to existing systems that are already built on top of relational databases which have normalization.</p> <p>"But normalization is a <strong>good</strong> thing, right?" Well, if you are using a relational database yes. "But why does that matter if I am using EF?" A common "normalized" table is Address. Possible scenario: Company (location of business/office) and Contact (might be "remote" worker so they are not at the business location) and they both have a FK that points to Address. Using one edmx file for Company and one for Contact (even with different namespaces) that both include the Address table, the code will compile but at run time you will get this beauty:</p> <pre><code>Multiple types with the name 'Address' exist in the EdmItemCollection in different namespaces. Convention based mapping requires unique names without regard to namespace in the EdmItemCollection </code></pre> <p>You can change the mapping that is used by EF but then you have other "issues" when working through implementation and most people use the default mapping so forums like this won't have many pertinent questions and answers.</p> <p>You could also rename the Model name for the Address table to "ContactAddress" and "CompanyAddress" respectively, but that gives the illusion that they are different types when they really aren't. OK, so they <strong>are</strong> different types in EF but not in the database and, as I said, most of us "live" in the world of tacking on EF to an existing system with an existing data store that is a relational database.</p> <p>This is already a long-winded "answer" so I will stop here. I just wanted to make sure that people who landed here because they searched for "multiple edmx" and did not realize that there are significant difference between EF4 and EF5 were made aware and realized they may need to do some more investigating.</p>
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    1. POEntity Framework 4: Does it make sense to create a single diagram for all entities?
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