If you want to store a pickled object, you'll need to use a blob, since it is binary data. However, you can, say, base64 encode the pickled object to get a string that can be stored in a text field.
Generally, though, doing this sort of thing is indicative of bad design, since you're storing opaque data you lose the ability to use SQL to do any useful manipulation on that data. Although without knowing what you're actually doing, I can't really make a moral call on it.
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