Feature request: helpful error message on importing records with mix of null and non-null ids

[bblack]

I've just spent a fair amount of time tracking down an issue that manifested in our app as

PG::NotNullViolation: ERROR:  null value in column "id" violates not-null constraint

This was surprising, as I expect this gem to either

• not specify an id in the query, allowing postgres (in our case) to use the default column value (which is given by a seq), or
• specify an id with an expression referring to that sequence, as it usually does

@jkowens notes in another issue that

you cannot mix records that do not have an id set with those that do because the resulting sql statement will be invalid

This seems like an easy condition to detect, and possible actions include

• fill in unspecified id values with the "next sequence value" expression
• raise an error with a more helpful message

[doutatsu]

I've just experienced this issue and took me quite a bit of time to realise what was going on. So 👍🏻 from me as well, there should be a nicer error message to help with debugging