This is good advice, but there is two ways to go about it:
What normally happens, esp for employees (rather than 1099'ers) is that you disclose that you are working on your social network for farm animals (or whatever) and your employer in the CRM space agrees a clause that says that all work you do in the "farm animal social networking" space is yours.
However, you've a) had to disclose what you are specifically working on and b) this doesn't cover you if you then subsequently discover or design something in another space entirely.
Rather than agreeing all IP is your employer's and then jointly negating your own project, what would be better would be a closer ring fencing of what is theirs and you keeping control of everything else.
You would still need to declare any conflicts of interest, but I doubt a social network for farm animals would be a conflict for a CRM server and thus you might never need to disclose.
All of the above requires some careful lawyering and also assumes that you have the leverage to write/re-write your employment contract. Ideally this is best when you are negotiating at the start of the job.
But never assume that just cos an employer gives you are 'boiler plate' contract you can't cross it out and redefine your own terms - regardless of the size of the company. It just depends on your ability to negotiate and leverage.
tl;dr: don't ring fence out your IP, ring-fence out your employer's IP and own the rest.
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UPDATE: not everyone in life comes up with startup ideas and needs to consider IP issues. If you are that kind of person, which it sounds like you are, then consider whether being an employee makes sense for you at all (it doesn't for me for these reasons, which is why I've not been once since 2006).
Don't be afraid to take the same job but negotiate a 1099 contract instead which gives you greater freedoms (although you need to talk to your accountant to make sure your total business activities ensure the IRS agrees you are a 1099 status and not a W2). If a company needs a developer and you are good they shouldn't care what the paperwork looks like for you.