---
title: "Data migration and mapping"
description: "Data migration and mapping - URL: https://chatgpt.com/c/6a6e0125-5484-83ea-8d57-6a682c7900fc - Created: 2026-08-01T14:23:17.377Z --- So wait, so you have people submitting data. So it's like CSVs or…"
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last_updated: 2026-10-17
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---
# Data migration and mapping
Data migration and mapping - URL: https://chatgpt.com/c/6a6e0125-5484-83ea-8d57-6a682c7900fc - Created: 2026-08-01T14:23:17.377Z --- > So wait, so you have people submitting data. So it's like CSVs or…
## Content
# Data migration and mapping
- URL: https://chatgpt.com/c/6a6e0125-5484-83ea-8d57-6a682c7900fc
- Created: 2026-08-01T14:23:17.377Z
---
> So wait, so you have people submitting data. So it's like CSVs or JSON of all their data from their business. We take that and then we, so right now the way they do it, it takes months all manually. What I'm building right now is to basically one shot, attempt to one shot it, or not one shot it, but get it done automatically, and then just have the human review it. So they give us, they have like their members, their billing, their classes, their points, all that, and I'm like taking that, and it comes from, because we have like dozens of competitors, and we have a lot of people come from competitors. Dozens like millions. Oh right, yeah, yeah. But the top, the top ones that we do like are like, you know, just a handful. So we take those structures and I'm trying to like figure out the best way to map it. Yeah, I can share.
What you're describing is a data migration and schema mapping system, not just an ETL pipeline.
The core problem is:
- Input: CSVs
---
> Invent an elegant fluent builder Js api that is a composable grammar for expressing these transformations with opinionated defaults but can be customized by passing lambda . Show some examples (martial arts studio, cross fit gyms, etc…) to our software : class management, billing , crm, etc…
>
> Only examples from simple to more complex, input csv output structure and js code
💭 Thinking
**Planning migration examples**
I’m designing progressively complex JavaScript migration examples for martial arts, CrossFit, and CRM data, showing CSV inputs, structured outputs, composable transformations, joins, validation, deduplication, and customizable hooks.
**Planned migration examples**
{
key: unknown;
rows: Row[];
}
```
## `.from(generator)`
Creates entities using a custom iterable.
```ts
from(
generator:
| OutputRow[]
| Promise
| ((ctx: MigrationContext) =>
OutputRow[] | Promise)
): EntityBuilder;
```
---
# 11. Entity Keys
Every output entity should have a stable key.
## `.key(column)`
```ts
key(sourceColumn: string): EntityBuilder;
```
```js
.key("Member ID")
```
## `.key(lambda)`
```ts
key(
resolver: (
row: SourceRow,
ctx: RowContext
) => Primitive
): EntityBuilder;
```
```js
.key(row =>
`${row["Member ID"]}:${row["Plan Code"]}`
)
```
Keys are used for:
- Idempotency
- Conflict detection
- Entity references
- Import previews
- Update matching
- Error reporting
Unless configured otherwise, the output entity identifier is:
```txt
{targetType}:{key}
```
Example:
```txt
member:M-100
membership:C-100:UNLIMITED
```
---
# 12. Field Mapping
## `.field(path)`
Begins a field definition.
```ts
field(path: string): FieldBuilder;
```
Nested paths are supported.
```js
.field("billing.amount")
.field("address.city")
.field("migration.source")
```
---
## `.from(column)`
Copies a source value.
```ts
from(sourcePath: string): FieldBuilder;
```
```js
.field("email")
.from("Email")
```
Nested joined values may also be referenced:
```js
.field("planName")
.from("plan.Plan Name")
```
---
## `.value(value)`
Assigns a constant.
```ts
value(
value: unknown
): FieldBuilder;
```
```js
.field("currency")
.value("USD")
```
A lambda may be passed for dynamic constants:
```js
.field("importedAt")
.value(ctx => ctx.startedAt)
```
---
## `.map(transform)`
Transforms the current field value.
```ts
map(
transform: (
value: Input,
row: SourceRow,
ctx: RowContext
) => Output | Promise