Why did you say that can’t be right? You shouldn’t have to change RV dims that were already present in the original model, even if not all are used for posterior predictive

Ok, so “id” stays [0,1], but then new_idx must have to change to [1, 1, 2, 3] if we want to predict on one previous group (1) and two new groups (2, 3) correct?

So for example this code works

```
# Predict two new groups
new_idx = [1, 1, 2, 3]
new_coords = {
"id":[0, 1],
"new_id": [2, 3],
"obs_idx": list(range(len(new_idx))),
}
with pm.Model(coords=new_coords) as pred_m:
x = pm.Data("x", [0, 0, 0, 0], dims=("obs_idx",))
mu_group = pm.Normal("mu_group")
# Needs a new name, so old b is not used!
new_b = pm.Normal("new_b", mu_group, dims=("new_id",))
b = pm.Normal("b", mu_group, dims=("id",))
mu = pm.math.concatenate([new_b[new_idx], b[idx]]) * x
obs = pm.Normal("obs", mu, dims=("obs_idx",))
idata = pm.sample_posterior_predictive(idata, var_names=["obs"], predictions=True, extend_inferencedata=True)
```

Indexing should work just like numpy. You can either have zero-based indexing for both old and new groups (and ignore as many of either as you want), or you can have zero-based indexing for both groups, if you concatenate the `new_b`

and `b`

first and then index, instead of the other way around.

Your last example is odd, because you have a `new_idx`

with values 1 to 3 on `new_b`

which only has two entries. Sounds like you want the concatenate first and then index logic?

I’m more confused than ever. How should I write the prediction model if I want to predict on 1 of the previous groups and 2 new groups?

What you were doing was in the right direction. Just the indexing part seemed to be wrong that’s all. Here is how I would think it would work:

```
# Predict two new groups
new_idx = [1, 1, 2, 3]
new_coords = {
"id":[0, 1],
"new_id": [2, 3],
"obs_idx": list(range(len(new_idx))),
}
with pm.Model(coords=new_coords) as pred_m:
x = pm.Data("x", [0, 0, 0, 0], dims=("obs_idx",))
mu_group = pm.Normal("mu_group")
# Needs a new name, so old b is not used!
new_b = pm.Normal("new_b", mu_group, dims=("new_id",))
b = pm.Normal("b", mu_group, dims=("id",))
mu = pm.math.concatenate([b, new_b])[new_idx] * x
obs = pm.Normal("obs", mu, dims=("obs_idx",))
idata = pm.sample_posterior_predictive(idata, var_names=["obs"], predictions=True, extend_inferencedata=True)
```

The only line I changed was `mu = pm.math.concatenate([b, new_b])[new_idx] * x`

because you were doing invalid index with `new_idx`

.

The concatenate may be wrong how I did it, but the ideas is to get all the `b`

in a single vector.