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So, my work history is winding. I have worked at 1 job for 3 years as an entry level position. Last year I graduated college and moved into the field I got a degree in. I kept my job at the company in a part time fashion, usually 1-2 days a week. I didn't see it as a big woop di doo as I'd gotten used to being very busy. I also aquirred another job in my field as a developer.
Last august I was offered a promotion at my first job in order to entice me back however the salary didn't compare to my full time job so I declined, but have continued to work there. At the same time my full time job was providing shakily steady work and I feared being laid off so I quit and got another full time job as a developer for a second company.
In the end I see I made the right choice as after I left there was no more work for the rest of the year and I doubt my boss would have paid me to twiddle my thumbs, though he's never outright said he'd have let me go.
In January my 1st developer job called me stating that they'd landed a huge client and wanted me to spear head the project elevating me from a junior to senior developer and placing another junior dev under me. I jumped at the opportunity and we are now months into the project that I have sole control over. Here's the kicker, my part time job that I've kept all this time has now had a very senior position open and they are offering me the job. It comes with amazing pay, great benefits, and is much closer to home. I'd also be working with people I've known for years and doing something I truly love. Any other time I'd jump on this like a shark.
Now the catch, my current dev job is not something I can leave without lasting consequences. Not only am I the lead dev, but none of the other devs write in the language I use. The junior dev under me is in no way ready to take on a project like this. He is constantly in need of help writing even beginner code. If I leave I could end up costing my employer this client contract as I doubt he'll be able to deliver on time, even if he manages to replace me within the month, the dead line is literally a couple months away.
If I do decide to take the new job how should I approach my current boss?