Building data-driven web apps with Pyramid and SQLAlchemy Transcripts
Chapter: Database migrations with Alembic
Lecture: Concept: Alembic - the manual version
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We've seen there are two ways
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to make Alembic do changes to our database.
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We have the SQLAlchemy class based changes
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and just straight up manual changes.
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I didn't show you the manual changes
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but I did tell you about them.
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I'll show them to you now
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so you can see what the whole set
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of steps are and appreciate a little
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bit better what the SQLAlchemy version
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the AutoGen version is doing for you.
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If we're going to make a change
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we're going to record our revision.
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The manual way we're going to get started
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by creating the pieces by running
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Alembic revision and then give it a comment
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-m, add key words to the column.
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This is important, it's going to show up
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when you apply it. It's also going to show up in the file name
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that stores it. So, we do that and it'll generate some
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hash and then a file name friendly version of your comment.
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Done. Then we go and open that file and we see
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it looks like this. It has a revision, the first one will have no
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down revision, no relationship, but as you
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build upon them, of course, they do.
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And then it has an empty upgrade
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and an empty pass. So, your job in the manual version
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is to actually implement those things.
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Okay, that may or may not be something
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you want to do. It's not too hard.
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Here's another one we can do.
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We can come and say we'd like to do op
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add column to the packages table and just passing
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a column like you would in your SQLAlchemy class, right?
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I say, the column, keywords, string
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nullable, and so on, then downgrade of course
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is drop column. This is pretty straight forward
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but it can be a little bit tricky.
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One thing you may want to do is
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double check that this column does not exist
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before you try to add it.
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If it does, this migration will fail.
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And everything that depends upon it
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will subsequently fail as well.
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And that won't be great.
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Similarly for dropping it.
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I talked a little bit earlier about
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the alembic_helpers.
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There's this function that I got called
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table has column.
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And give it a table and a column
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it'll say yes, I have it.
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So, it'll go and do some funky introspection
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on the table and tell you whether
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or not that it's there.
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The way you import it is a little weird.
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Because where we run the code from
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and where it lives is not exactly
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the same place. The way Alembic runs these
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it's a little bit funky.
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So, you got to do this weird import
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to get it, but once you do then you can ask
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questions like, if the table doesn't have
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a keyword's column, then add it.
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Things like that.
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So, this may be helpful to you.
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We could also ask alembic current.
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By default it'll just say nothing
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which means there's no version.
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Although it does when you run that create table
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just there's no version put into it.
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And then we say upgrade
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and it's going to apply whatever upgrade.
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So, in this case, we ran it on the database
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that had revision 708 and we upgraded a 689.
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And that was adding the keywords column.
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Now, it you ask Alembic.current
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it'll say, we're on that 689 version
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that we just upgraded to and by the way
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that is the latest.
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And of course, as we saw, this version
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is stored in your database so you have
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an Alembic.version and you go do a query
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on it, you'll see 689, the one column
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with one row that is just that string.
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Super easy to check.
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Might just open it up in your tools
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and going what version is this in?
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Even if you can't Alembic directly on it.
