A common error in analysing an elaborate simile is extracting only a fragment of its intended point. The remaining comparison may then lose or change its force.
From the books-and-donkey example, choosing only “carrying a load” discards the failure to benefit intellectually. Choosing only “ignorance” ignores the more pointed situation of having the very materials of learning at hand.
Yet one should not declare every long expression composite by default. The task is to identify which constituent relations are necessary for the intended comparison, and include those rather than every accidental detail.